<!-- doc/src/sgml/system-views.sgml -->
<!--
 Documentation of the system views, directed toward PostgreSQL developers
 -->

<chapter id="views">
 <title>System Views</title>

  <para>
   In addition to the system catalogs, <productname>PostgreSQL</productname>
   provides a number of built-in views.  Some system views provide convenient
   access to some commonly used queries on the system catalogs.  Other views
   provide access to internal server state.
  </para>

  <para>
   The information schema (<xref linkend="information-schema"/>) provides
   an alternative set of views which overlap the functionality of the system
   views.  Since the information schema is SQL-standard whereas the views
   described here are <productname>PostgreSQL</productname>-specific,
   it's usually better to use the information schema if it provides all
   the information you need.
  </para>

  <para>
   <xref linkend="view-table"/> lists the system views described here.
   More detailed documentation of each view follows below.
   There are some additional views that provide access to accumulated
   statistics; they are described in
   <xref linkend="monitoring-stats-views-table"/>.
  </para>

 <sect1 id="views-overview">
  <title>Overview</title>

  <para>
   <xref linkend="view-table"/> lists the system views.
   More detailed documentation of each catalog follows below.
   Except where noted, all the views described here are read-only.
  </para>

  <table id="view-table">
   <title>System Views</title>

   <tgroup cols="2">
    <thead>
     <row>
      <entry>View Name</entry>
      <entry>Purpose</entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry><link linkend="view-pg-available-extensions"><structname>pg_available_extensions</structname></link></entry>
      <entry>available extensions</entry>
     </row>

     <row>
      <entry><link linkend="view-pg-available-extension-versions"><structname>pg_available_extension_versions</structname></link></entry>
      <entry>available versions of extensions</entry>
     </row>

     <row>
      <entry><link linkend="view-pg-backend-memory-contexts"><structname>pg_backend_memory_contexts</structname></link></entry>
      <entry>backend memory contexts</entry>
     </row>

     <row>
      <entry><link linkend="view-pg-config"><structname>pg_config</structname></link></entry>
      <entry>compile-time configuration parameters</entry>
     </row>

     <row>
      <entry><link linkend="view-pg-cursors"><structname>pg_cursors</structname></link></entry>
      <entry>open cursors</entry>
     </row>

     <row>
      <entry><link linkend="view-pg-file-settings"><structname>pg_file_settings</structname></link></entry>
      <entry>summary of configuration file contents</entry>
     </row>

     <row>
      <entry><link linkend="view-pg-group"><structname>pg_group</structname></link></entry>
      <entry>groups of database users</entry>
     </row>

     <row>
      <entry><link linkend="view-pg-hba-file-rules"><structname>pg_hba_file_rules</structname></link></entry>
      <entry>summary of client authentication configuration file contents</entry>
     </row>

     <row>
      <entry><link linkend="view-pg-ident-file-mappings"><structname>pg_ident_file_mappings</structname></link></entry>
      <entry>summary of client user name mapping configuration file contents</entry>
     </row>

     <row>
      <entry><link linkend="view-pg-indexes"><structname>pg_indexes</structname></link></entry>
      <entry>indexes</entry>
     </row>

     <row>
      <entry><link linkend="view-pg-locks"><structname>pg_locks</structname></link></entry>
      <entry>locks currently held or awaited</entry>
     </row>

     <row>
      <entry><link linkend="view-pg-matviews"><structname>pg_matviews</structname></link></entry>
      <entry>materialized views</entry>
     </row>

     <row>
      <entry><link linkend="view-pg-policies"><structname>pg_policies</structname></link></entry>
      <entry>policies</entry>
     </row>

     <row>
      <entry><link linkend="view-pg-prepared-statements"><structname>pg_prepared_statements</structname></link></entry>
      <entry>prepared statements</entry>
     </row>

     <row>
      <entry><link linkend="view-pg-prepared-xacts"><structname>pg_prepared_xacts</structname></link></entry>
      <entry>prepared transactions</entry>
     </row>

     <row>
      <entry><link linkend="view-pg-publication-tables"><structname>pg_publication_tables</structname></link></entry>
      <entry>publications and information of their associated tables</entry>
     </row>

     <row>
      <entry><link linkend="view-pg-replication-origin-status"><structname>pg_replication_origin_status</structname></link></entry>
      <entry>information about replication origins, including replication progress</entry>
     </row>

     <row>
      <entry><link linkend="view-pg-replication-slots"><structname>pg_replication_slots</structname></link></entry>
      <entry>replication slot information</entry>
     </row>

     <row>
      <entry><link linkend="view-pg-roles"><structname>pg_roles</structname></link></entry>
      <entry>database roles</entry>
     </row>

     <row>
      <entry><link linkend="view-pg-rules"><structname>pg_rules</structname></link></entry>
      <entry>rules</entry>
     </row>

     <row>
      <entry><link linkend="view-pg-seclabels"><structname>pg_seclabels</structname></link></entry>
      <entry>security labels</entry>
     </row>

     <row>
      <entry><link linkend="view-pg-sequences"><structname>pg_sequences</structname></link></entry>
      <entry>sequences</entry>
     </row>

     <row>
      <entry><link linkend="view-pg-settings"><structname>pg_settings</structname></link></entry>
      <entry>parameter settings</entry>
     </row>

     <row>
      <entry><link linkend="view-pg-shadow"><structname>pg_shadow</structname></link></entry>
      <entry>database users</entry>
     </row>

     <row>
      <entry><link linkend="view-pg-shmem-allocations"><structname>pg_shmem_allocations</structname></link></entry>
      <entry>shared memory allocations</entry>
     </row>

     <row>
      <entry><link linkend="view-pg-stats"><structname>pg_stats</structname></link></entry>
      <entry>planner statistics</entry>
     </row>

     <row>
      <entry><link linkend="view-pg-stats-ext"><structname>pg_stats_ext</structname></link></entry>
      <entry>extended planner statistics</entry>
     </row>

     <row>
      <entry><link linkend="view-pg-stats-ext-exprs"><structname>pg_stats_ext_exprs</structname></link></entry>
      <entry>extended planner statistics for expressions</entry>
     </row>

     <row>
      <entry><link linkend="view-pg-tables"><structname>pg_tables</structname></link></entry>
      <entry>tables</entry>
     </row>

     <row>
      <entry><link linkend="view-pg-timezone-abbrevs"><structname>pg_timezone_abbrevs</structname></link></entry>
      <entry>time zone abbreviations</entry>
     </row>

     <row>
      <entry><link linkend="view-pg-timezone-names"><structname>pg_timezone_names</structname></link></entry>
      <entry>time zone names</entry>
     </row>

     <row>
      <entry><link linkend="view-pg-user"><structname>pg_user</structname></link></entry>
      <entry>database users</entry>
     </row>

     <row>
      <entry><link linkend="view-pg-user-mappings"><structname>pg_user_mappings</structname></link></entry>
      <entry>user mappings</entry>
     </row>

     <row>
      <entry><link linkend="view-pg-views"><structname>pg_views</structname></link></entry>
      <entry>views</entry>
     </row>

     <row>
      <entry><link linkend="view-pg-wait-events"><structname>pg_wait_events</structname></link></entry>
      <entry>wait events</entry>
     </row>

    </tbody>
   </tgroup>
  </table>
 </sect1>

 <sect1 id="view-pg-available-extensions">
  <title><structname>pg_available_extensions</structname></title>

  <indexterm zone="view-pg-available-extensions">
   <primary>pg_available_extensions</primary>
  </indexterm>

  <para>
   The <structname>pg_available_extensions</structname> view lists the
   extensions that are available for installation.
   See also the
   <link linkend="catalog-pg-extension"><structname>pg_extension</structname></link>
   catalog, which shows the extensions currently installed.
  </para>

  <table>
   <title><structname>pg_available_extensions</structname> Columns</title>
   <tgroup cols="1">
    <thead>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       Column Type
      </para>
      <para>
       Description
      </para></entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>name</structfield> <type>name</type>
      </para>
      <para>
       Extension name
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>default_version</structfield> <type>text</type>
      </para>
      <para>
       Name of default version, or <literal>NULL</literal> if none is
       specified
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>installed_version</structfield> <type>text</type>
      </para>
      <para>
       Currently installed version of the extension,
       or <literal>NULL</literal> if not installed
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>comment</structfield> <type>text</type>
      </para>
      <para>
       Comment string from the extension's control file
      </para></entry>
     </row>
    </tbody>
   </tgroup>
  </table>

  <para>
   The <structname>pg_available_extensions</structname> view is read-only.
  </para>
 </sect1>

 <sect1 id="view-pg-available-extension-versions">
  <title><structname>pg_available_extension_versions</structname></title>

  <indexterm zone="view-pg-available-extension-versions">
   <primary>pg_available_extension_versions</primary>
  </indexterm>

  <para>
   The <structname>pg_available_extension_versions</structname> view lists the
   specific extension versions that are available for installation.
   See also the <link
   linkend="catalog-pg-extension"><structname>pg_extension</structname></link>
   catalog, which shows the extensions currently installed.
  </para>

  <table>
   <title><structname>pg_available_extension_versions</structname> Columns</title>
   <tgroup cols="1">
    <thead>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       Column Type
      </para>
      <para>
       Description
      </para></entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>name</structfield> <type>name</type>
      </para>
      <para>
       Extension name
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>version</structfield> <type>text</type>
      </para>
      <para>
       Version name
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>installed</structfield> <type>bool</type>
      </para>
      <para>
       True if this version of this extension is currently
       installed
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>superuser</structfield> <type>bool</type>
      </para>
      <para>
       True if only superusers are allowed to install this extension
       (but see <structfield>trusted</structfield>)
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>trusted</structfield> <type>bool</type>
      </para>
      <para>
       True if the extension can be installed by non-superusers
       with appropriate privileges
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>relocatable</structfield> <type>bool</type>
      </para>
      <para>
       True if extension can be relocated to another schema
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>schema</structfield> <type>name</type>
      </para>
      <para>
       Name of the schema that the extension must be installed into,
       or <literal>NULL</literal> if partially or fully relocatable
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>requires</structfield> <type>name[]</type>
      </para>
      <para>
       Names of prerequisite extensions,
       or <literal>NULL</literal> if none
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>comment</structfield> <type>text</type>
      </para>
      <para>
       Comment string from the extension's control file
      </para></entry>
     </row>
    </tbody>
   </tgroup>
  </table>

  <para>
   The <structname>pg_available_extension_versions</structname> view is
   read-only.
  </para>
 </sect1>

 <sect1 id="view-pg-backend-memory-contexts">
  <title><structname>pg_backend_memory_contexts</structname></title>

  <indexterm zone="view-pg-backend-memory-contexts">
   <primary>pg_backend_memory_contexts</primary>
  </indexterm>

  <para>
   The view <structname>pg_backend_memory_contexts</structname> displays all
   the memory contexts of the server process attached to the current session.
  </para>
  <para>
   <structname>pg_backend_memory_contexts</structname> contains one row
   for each memory context.
  </para>

  <table>
   <title><structname>pg_backend_memory_contexts</structname> Columns</title>
   <tgroup cols="1">
    <thead>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       Column Type
      </para>
      <para>
       Description
      </para></entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>name</structfield> <type>text</type>
      </para>
      <para>
       Name of the memory context
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>ident</structfield> <type>text</type>
      </para>
      <para>
       Identification information of the memory context. This field is truncated at 1024 bytes
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>type</structfield> <type>text</type>
      </para>
      <para>
       Type of the memory context
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>level</structfield> <type>int4</type>
      </para>
      <para>
       The 1-based level of the context in the memory context hierarchy. The
       level of a context also shows the position of that context in the
       <structfield>path</structfield> column.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>path</structfield> <type>int4[]</type>
      </para>
      <para>
       Array of transient numerical identifiers to describe the memory
       context hierarchy. The first element is for
       <literal>TopMemoryContext</literal>, subsequent elements contain
       intermediate parents and the final element contains the identifier for
       the current context.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>total_bytes</structfield> <type>int8</type>
      </para>
      <para>
       Total bytes allocated for this memory context
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>total_nblocks</structfield> <type>int8</type>
      </para>
      <para>
       Total number of blocks allocated for this memory context
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>free_bytes</structfield> <type>int8</type>
      </para>
      <para>
       Free space in bytes
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>free_chunks</structfield> <type>int8</type>
      </para>
      <para>
       Total number of free chunks
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>used_bytes</structfield> <type>int8</type>
      </para>
      <para>
       Used space in bytes
      </para></entry>
     </row>
    </tbody>
   </tgroup>
  </table>

  <para>
   By default, the <structname>pg_backend_memory_contexts</structname> view can be
   read only by superusers or roles with the privileges of the
   <literal>pg_read_all_stats</literal> role.
  </para>

  <para>
   Since memory contexts are created and destroyed during the running of a
   query, the identifiers stored in the <structfield>path</structfield> column
   can be unstable between multiple invocations of the view in the same query.
   The example below demonstrates an effective usage of this column and
   calculates the total number of bytes used by
   <literal>CacheMemoryContext</literal> and all of its children:

<programlisting>
WITH memory_contexts AS (
    SELECT * FROM pg_backend_memory_contexts
)
SELECT sum(c1.total_bytes)
FROM memory_contexts c1, memory_contexts c2
WHERE c2.name = 'CacheMemoryContext'
AND c1.path[c2.level] = c2.path[c2.level];
</programlisting>

   The <link linkend="queries-with">Common Table Expression</link> is used
   to ensure the context IDs in the <structfield>path</structfield> column
   match between both evaluations of the view.
  </para>
 </sect1>

 <sect1 id="view-pg-config">
  <title><structname>pg_config</structname></title>

  <indexterm zone="view-pg-config">
   <primary>pg_config</primary>
  </indexterm>

  <para>
   The view <structname>pg_config</structname> describes the
   compile-time configuration parameters of the currently installed
   version of <productname>PostgreSQL</productname>. It is intended, for example, to
   be used by software packages that want to interface to
   <productname>PostgreSQL</productname> to facilitate finding the required header
   files and libraries. It provides the same basic information as the
   <xref linkend="app-pgconfig"/> <productname>PostgreSQL</productname> client
   application.
  </para>

  <para>
   By default, the <structname>pg_config</structname> view can be read
   only by superusers.
  </para>

  <table>
   <title><structname>pg_config</structname> Columns</title>
   <tgroup cols="1">
    <thead>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       Column Type
      </para>
      <para>
       Description
      </para></entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>name</structfield> <type>text</type>
      </para>
      <para>
       The parameter name
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>setting</structfield> <type>text</type>
      </para>
      <para>
       The parameter value
      </para></entry>
     </row>
    </tbody>
   </tgroup>
  </table>

 </sect1>

 <sect1 id="view-pg-cursors">
  <title><structname>pg_cursors</structname></title>

  <indexterm zone="view-pg-cursors">
   <primary>pg_cursors</primary>
  </indexterm>

  <para>
   The <structname>pg_cursors</structname> view lists the cursors that
   are currently available. Cursors can be defined in several ways:
   <itemizedlist>
    <listitem>
     <para>
      via the <link linkend="sql-declare"><command>DECLARE</command></link>
      statement in SQL
     </para>
    </listitem>

    <listitem>
     <para>
      via the Bind message in the frontend/backend protocol, as
      described in <xref linkend="protocol-flow-ext-query"/>
     </para>
    </listitem>

    <listitem>
     <para>
      via the Server Programming Interface (SPI), as described in
      <xref linkend="spi-interface"/>
     </para>
    </listitem>
   </itemizedlist>

   The <structname>pg_cursors</structname> view displays cursors
   created by any of these means. Cursors only exist for the duration
   of the transaction that defines them, unless they have been
   declared <literal>WITH HOLD</literal>. Therefore non-holdable
   cursors are only present in the view until the end of their
   creating transaction.

   <note>
    <para>
     Cursors are used internally to implement some of the components
     of <productname>PostgreSQL</productname>, such as procedural languages.
     Therefore, the <structname>pg_cursors</structname> view might include cursors
     that have not been explicitly created by the user.
    </para>
   </note>
  </para>

  <table>
   <title><structname>pg_cursors</structname> Columns</title>
   <tgroup cols="1">
    <thead>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       Column Type
      </para>
      <para>
       Description
      </para></entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>name</structfield> <type>text</type>
      </para>
      <para>
       The name of the cursor
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>statement</structfield> <type>text</type>
      </para>
      <para>
       The verbatim query string submitted to declare this cursor
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>is_holdable</structfield> <type>bool</type>
      </para>
      <para>
       <literal>true</literal> if the cursor is holdable (that is, it
       can be accessed after the transaction that declared the cursor
       has committed); <literal>false</literal> otherwise
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>is_binary</structfield> <type>bool</type>
      </para>
      <para>
       <literal>true</literal> if the cursor was declared
       <literal>BINARY</literal>; <literal>false</literal>
       otherwise
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>is_scrollable</structfield> <type>bool</type>
      </para>
      <para>
       <literal>true</literal> if the cursor is scrollable (that is, it
       allows rows to be retrieved in a nonsequential manner);
       <literal>false</literal> otherwise
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>creation_time</structfield> <type>timestamptz</type>
      </para>
      <para>
       The time at which the cursor was declared
      </para></entry>
     </row>
    </tbody>
   </tgroup>
  </table>

  <para>
   The <structname>pg_cursors</structname> view is read-only.
  </para>

 </sect1>

 <sect1 id="view-pg-file-settings">
  <title><structname>pg_file_settings</structname></title>

  <indexterm zone="view-pg-file-settings">
   <primary>pg_file_settings</primary>
  </indexterm>

  <para>
   The view <structname>pg_file_settings</structname> provides a summary of
   the contents of the server's configuration file(s).  A row appears in
   this view for each <quote>name = value</quote> entry appearing in the files,
   with annotations indicating whether the value could be applied
   successfully.  Additional row(s) may appear for problems not linked to
   a <quote>name = value</quote> entry, such as syntax errors in the files.
  </para>

  <para>
   This view is helpful for checking whether planned changes in the
   configuration files will work, or for diagnosing a previous failure.
   Note that this view reports on the <emphasis>current</emphasis> contents of the
   files, not on what was last applied by the server.  (The
   <link linkend="view-pg-settings"><structname>pg_settings</structname></link>
   view is usually sufficient to determine that.)
  </para>

  <para>
   By default, the <structname>pg_file_settings</structname> view can be read
   only by superusers.
  </para>

  <table>
   <title><structname>pg_file_settings</structname> Columns</title>
   <tgroup cols="1">
    <thead>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       Column Type
      </para>
      <para>
       Description
      </para></entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>sourcefile</structfield> <type>text</type>
      </para>
      <para>
       Full path name of the configuration file
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>sourceline</structfield> <type>int4</type>
      </para>
      <para>
       Line number within the configuration file where the entry appears
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>seqno</structfield> <type>int4</type>
      </para>
      <para>
       Order in which the entries are processed (1..<replaceable>n</replaceable>)
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>name</structfield> <type>text</type>
      </para>
      <para>
       Configuration parameter name
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>setting</structfield> <type>text</type>
      </para>
      <para>
       Value to be assigned to the parameter
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>applied</structfield> <type>bool</type>
      </para>
      <para>
       True if the value can be applied successfully
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>error</structfield> <type>text</type>
      </para>
      <para>
       If not null, an error message indicating why this entry could
       not be applied
      </para></entry>
     </row>
    </tbody>
   </tgroup>
  </table>

  <para>
   If the configuration file contains syntax errors or invalid parameter
   names, the server will not attempt to apply any settings from it, and
   therefore all the <structfield>applied</structfield> fields will read as false.
   In such a case there will be one or more rows with
   non-null <structfield>error</structfield> fields indicating the
   problem(s).  Otherwise, individual settings will be applied if possible.
   If an individual setting cannot be applied (e.g., invalid value, or the
   setting cannot be changed after server start) it will have an appropriate
   message in the <structfield>error</structfield> field.  Another way that
   an entry might have <structfield>applied</structfield> = false is that it is
   overridden by a later entry for the same parameter name; this case is not
   considered an error so nothing appears in
   the <structfield>error</structfield> field.
  </para>

  <para>
   See <xref linkend="config-setting"/> for more information about the various
   ways to change run-time parameters.
  </para>

</sect1>

 <sect1 id="view-pg-group">
  <title><structname>pg_group</structname></title>

  <indexterm zone="view-pg-group">
   <primary>pg_group</primary>
  </indexterm>

  <!-- Unlike information_schema.applicable_roles, this shows no members for
       pg_database_owner.  The v8.1 catalog would have shown no members if
       that role had existed at the time. -->
  <para>
   The view <structname>pg_group</structname> exists for backwards
   compatibility: it emulates a catalog that existed in
   <productname>PostgreSQL</productname> before version 8.1.
   It shows the names and members of all roles that are marked as not
   <structfield>rolcanlogin</structfield>, which is an approximation to the set
   of roles that are being used as groups.
  </para>

  <table>
   <title><structname>pg_group</structname> Columns</title>
   <tgroup cols="1">
    <thead>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       Column Type
      </para>
      <para>
       Description
      </para></entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>groname</structfield> <type>name</type>
       (references <link linkend="catalog-pg-authid"><structname>pg_authid</structname></link>.<structfield>rolname</structfield>)
      </para>
      <para>
       Name of the group
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>grosysid</structfield> <type>oid</type>
       (references <link linkend="catalog-pg-authid"><structname>pg_authid</structname></link>.<structfield>oid</structfield>)
      </para>
      <para>
       ID of this group
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>grolist</structfield> <type>oid[]</type>
       (references <link linkend="catalog-pg-authid"><structname>pg_authid</structname></link>.<structfield>oid</structfield>)
      </para>
      <para>
       An array containing the IDs of the roles in this group
      </para></entry>
     </row>
    </tbody>
   </tgroup>
  </table>

 </sect1>

 <sect1 id="view-pg-hba-file-rules">
  <title><structname>pg_hba_file_rules</structname></title>

  <indexterm zone="view-pg-hba-file-rules">
   <primary>pg_hba_file_rules</primary>
  </indexterm>

  <para>
   The view <structname>pg_hba_file_rules</structname> provides a summary of
   the contents of the client authentication configuration file,
   <link linkend="auth-pg-hba-conf"><filename>pg_hba.conf</filename></link>.
   A row appears in this view for each
   non-empty, non-comment line in the file, with annotations indicating
   whether the rule could be applied successfully.
  </para>

  <para>
   This view can be helpful for checking whether planned changes in the
   authentication configuration file will work, or for diagnosing a previous
   failure.  Note that this view reports on the <emphasis>current</emphasis> contents
   of the file, not on what was last loaded by the server.
  </para>

  <para>
   By default, the <structname>pg_hba_file_rules</structname> view can be read
   only by superusers.
  </para>

  <table>
   <title><structname>pg_hba_file_rules</structname> Columns</title>
   <tgroup cols="1">
    <thead>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       Column Type
      </para>
      <para>
       Description
      </para></entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>rule_number</structfield> <type>int4</type>
      </para>
      <para>
       Number of this rule, if valid, otherwise <literal>NULL</literal>.
       This indicates the order in which each rule is considered
       until a match is found during authentication.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>file_name</structfield> <type>text</type>
      </para>
      <para>
       Name of the file containing this rule
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>line_number</structfield> <type>int4</type>
      </para>
      <para>
       Line number of this rule in <literal>file_name</literal>
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>type</structfield> <type>text</type>
      </para>
      <para>
       Type of connection
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>database</structfield> <type>text[]</type>
      </para>
      <para>
       List of database name(s) to which this rule applies
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>user_name</structfield> <type>text[]</type>
      </para>
      <para>
       List of user and group name(s) to which this rule applies
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>address</structfield> <type>text</type>
      </para>
      <para>
       Host name or IP address, or one
       of <literal>all</literal>, <literal>samehost</literal>,
       or <literal>samenet</literal>, or null for local connections
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>netmask</structfield> <type>text</type>
      </para>
      <para>
       IP address mask, or null if not applicable
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>auth_method</structfield> <type>text</type>
      </para>
      <para>
       Authentication method
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>options</structfield> <type>text[]</type>
      </para>
      <para>
       Options specified for authentication method, if any
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>error</structfield> <type>text</type>
      </para>
      <para>
       If not null, an error message indicating why this
       line could not be processed
      </para></entry>
     </row>
    </tbody>
   </tgroup>
  </table>

  <para>
   Usually, a row reflecting an incorrect entry will have values for only
   the <structfield>line_number</structfield> and <structfield>error</structfield> fields.
  </para>

  <para>
   See <xref linkend="client-authentication"/> for more information about
   client authentication configuration.
  </para>
 </sect1>

 <sect1 id="view-pg-ident-file-mappings">
  <title><structname>pg_ident_file_mappings</structname></title>

  <indexterm zone="view-pg-ident-file-mappings">
   <primary>pg_ident_file_mappings</primary>
  </indexterm>

  <para>
   The view <structname>pg_ident_file_mappings</structname> provides a summary
   of the contents of the client user name mapping configuration file,
   <link linkend="auth-username-maps"><filename>pg_ident.conf</filename></link>.
   A row appears in this view for each non-empty, non-comment line in the file,
   with annotations indicating whether the map could be applied successfully.
  </para>

  <para>
   This view can be helpful for checking whether planned changes in the
   authentication configuration file will work, or for diagnosing a previous
   failure.  Note that this view reports on the <emphasis>current</emphasis>
   contents of the file, not on what was last loaded by the server.
  </para>

  <para>
   By default, the <structname>pg_ident_file_mappings</structname> view can be
   read only by superusers.
  </para>

  <table>
   <title><structname>pg_ident_file_mappings</structname> Columns</title> <tgroup
   cols="1">
    <thead>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       Column Type
      </para>
      <para>
       Description
      </para></entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>map_number</structfield> <type>int4</type>
      </para>
      <para>
       Number of this map, in priority order, if valid, otherwise
       <literal>NULL</literal>
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>file_name</structfield> <type>text</type>
      </para>
      <para>
       Name of the file containing this map
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>line_number</structfield> <type>int4</type>
      </para>
      <para>
       Line number of this map in <literal>file_name</literal>
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>map_name</structfield> <type>text</type>
      </para>
      <para>
       Name of the map
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>sys_name</structfield> <type>text</type>
      </para>
      <para>
       Detected user name of the client
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>pg_username</structfield> <type>text</type>
      </para>
      <para>
       Requested PostgreSQL user name
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>error</structfield> <type>text</type>
      </para>
      <para>
       If not <literal>NULL</literal>, an error message indicating why this
       line could not be processed
      </para></entry>
     </row>
    </tbody>
   </tgroup>
  </table>

  <para>
   Usually, a row reflecting an incorrect entry will have values for only
   the <structfield>line_number</structfield> and <structfield>error</structfield> fields.
  </para>

  <para>
   See <xref linkend="client-authentication"/> for more information about
   client authentication configuration.
  </para>
 </sect1>

 <sect1 id="view-pg-indexes">
  <title><structname>pg_indexes</structname></title>

  <indexterm zone="view-pg-indexes">
   <primary>pg_indexes</primary>
  </indexterm>

  <para>
   The view <structname>pg_indexes</structname> provides access to
   useful information about each index in the database.
  </para>

  <table>
   <title><structname>pg_indexes</structname> Columns</title>
   <tgroup cols="1">
    <thead>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       Column Type
      </para>
      <para>
       Description
      </para></entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>schemaname</structfield> <type>name</type>
       (references <link linkend="catalog-pg-namespace"><structname>pg_namespace</structname></link>.<structfield>nspname</structfield>)
      </para>
      <para>
       Name of schema containing table and index
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>tablename</structfield> <type>name</type>
       (references <link linkend="catalog-pg-class"><structname>pg_class</structname></link>.<structfield>relname</structfield>)
      </para>
      <para>
       Name of table the index is for
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>indexname</structfield> <type>name</type>
       (references <link linkend="catalog-pg-class"><structname>pg_class</structname></link>.<structfield>relname</structfield>)
      </para>
      <para>
       Name of index
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>tablespace</structfield> <type>name</type>
       (references <link linkend="catalog-pg-tablespace"><structname>pg_tablespace</structname></link>.<structfield>spcname</structfield>)
      </para>
      <para>
       Name of tablespace containing index (null if default for database)
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>indexdef</structfield> <type>text</type>
      </para>
      <para>
       Index definition (a reconstructed <xref linkend="sql-createindex"/>
       command)
      </para></entry>
     </row>
    </tbody>
   </tgroup>
  </table>

 </sect1>

 <sect1 id="view-pg-locks">
  <title><structname>pg_locks</structname></title>

  <indexterm zone="view-pg-locks">
   <primary>pg_locks</primary>
  </indexterm>

  <para>
   The view <structname>pg_locks</structname> provides access to
   information about the locks held by active processes within the
   database server.  See <xref linkend="mvcc"/> for more discussion
   of locking.
  </para>

  <para>
   <structname>pg_locks</structname> contains one row per active lockable
   object, requested lock mode, and relevant process.  Thus, the same
   lockable object might
   appear many times, if multiple processes are holding or waiting
   for locks on it.  However, an object that currently has no locks on it
   will not appear at all.
  </para>

  <para>
   There are several distinct types of lockable objects:
   whole relations (e.g., tables), individual pages of relations,
   individual tuples of relations,
   transaction IDs (both virtual and permanent IDs),
   and general database objects (identified by class OID and object OID,
   in the same way as in <link linkend="catalog-pg-description"><structname>pg_description</structname></link> or
   <link linkend="catalog-pg-depend"><structname>pg_depend</structname></link>).  Also, the right to extend a
   relation is represented as a separate lockable object, as is the right to
   update <structname>pg_database</structname>.<structfield>datfrozenxid</structfield>.
   Also, <quote>advisory</quote> locks can be taken on numbers that have
   user-defined meanings.
  </para>

  <table>
   <title><structname>pg_locks</structname> Columns</title>
   <tgroup cols="1">
    <thead>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       Column Type
      </para>
      <para>
       Description
      </para></entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>locktype</structfield> <type>text</type>
      </para>
      <para>
       Type of the lockable object:
       <literal>relation</literal>,
       <literal>extend</literal>,
       <literal>frozenid</literal>,
       <literal>page</literal>,
       <literal>tuple</literal>,
       <literal>transactionid</literal>,
       <literal>virtualxid</literal>,
       <literal>spectoken</literal>,
       <literal>object</literal>,
       <literal>userlock</literal>,
       <literal>advisory</literal>, or
       <literal>applytransaction</literal>.
       (See also <xref linkend="wait-event-lock-table"/>.)
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>database</structfield> <type>oid</type>
       (references <link linkend="catalog-pg-database"><structname>pg_database</structname></link>.<structfield>oid</structfield>)
      </para>
      <para>
       OID of the database in which the lock target exists, or
       zero if the target is a shared object, or
       null if the target is a transaction ID
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>relation</structfield> <type>oid</type>
       (references <link linkend="catalog-pg-class"><structname>pg_class</structname></link>.<structfield>oid</structfield>)
      </para>
      <para>
       OID of the relation targeted by the lock, or null if the target is not
       a relation or part of a relation
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>page</structfield> <type>int4</type>
      </para>
      <para>
       Page number targeted by the lock within the relation,
       or null if the target is not a relation page or tuple
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>tuple</structfield> <type>int2</type>
      </para>
      <para>
       Tuple number targeted by the lock within the page,
       or null if the target is not a tuple
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>virtualxid</structfield> <type>text</type>
      </para>
      <para>
       Virtual ID of the transaction targeted by the lock,
       or null if the target is not a virtual transaction ID;  see
       <xref linkend="transactions"/>
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>transactionid</structfield> <type>xid</type>
      </para>
      <para>
       ID of the transaction targeted by the lock, or null if the target
       is not a transaction ID;  <xref linkend="transactions"/>
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>classid</structfield> <type>oid</type>
       (references <link linkend="catalog-pg-class"><structname>pg_class</structname></link>.<structfield>oid</structfield>)
      </para>
      <para>
       OID of the system catalog containing the lock target, or null if the
       target is not a general database object
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>objid</structfield> <type>oid</type>
       (references any OID column)
      </para>
      <para>
       OID of the lock target within its system catalog, or null if the
       target is not a general database object
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>objsubid</structfield> <type>int2</type>
      </para>
      <para>
       Column number targeted by the lock (the
       <structfield>classid</structfield> and <structfield>objid</structfield> refer to the
       table itself),
       or zero if the target is some other general database object,
       or null if the target is not a general database object
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>virtualtransaction</structfield> <type>text</type>
      </para>
      <para>
       Virtual ID of the transaction that is holding or awaiting this lock
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>pid</structfield> <type>int4</type>
      </para>
      <para>
       Process ID of the server process holding or awaiting this
       lock, or null if the lock is held by a prepared transaction
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>mode</structfield> <type>text</type>
      </para>
      <para>
       Name of the lock mode held or desired by this process (see <xref linkend="locking-tables"/> and <xref linkend="xact-serializable"/>)
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>granted</structfield> <type>bool</type>
      </para>
      <para>
       True if lock is held, false if lock is awaited
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>fastpath</structfield> <type>bool</type>
      </para>
      <para>
       True if lock was taken via fast path, false if taken via main
       lock table
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>waitstart</structfield> <type>timestamptz</type>
      </para>
      <para>
       Time when the server process started waiting for this lock,
       or null if the lock is held.
       Note that this can be null for a very short period of time after
       the wait started even though <structfield>granted</structfield>
       is <literal>false</literal>.
      </para></entry>
     </row>
    </tbody>
   </tgroup>
  </table>

  <para>
   <structfield>granted</structfield> is true in a row representing a lock
   held by the indicated process.  False indicates that this process is
   currently waiting to acquire this lock, which implies that at least one
   other process is holding or waiting for a conflicting lock mode on the same
   lockable object.  The waiting process will sleep until the other lock is
   released (or a deadlock situation is detected).  A single process can be
   waiting to acquire at most one lock at a time.
  </para>

  <para>
   Throughout running a transaction, a server process holds an exclusive lock
   on the transaction's virtual transaction ID.  If a permanent ID is assigned
   to the transaction (which normally happens only if the transaction changes
   the state of the database), it also holds an exclusive lock on the
   transaction's permanent transaction ID until it ends.  When a process finds
   it necessary to wait specifically for another transaction to end, it does
   so by attempting to acquire share lock on the other transaction's ID
   (either virtual or permanent ID depending on the situation). That will
   succeed only when the other transaction terminates and releases its locks.
  </para>

  <para>
   Although tuples are a lockable type of object,
   information about row-level locks is stored on disk, not in memory,
   and therefore row-level locks normally do not appear in this view.
   If a process is waiting for a
   row-level lock, it will usually appear in the view as waiting for the
   permanent transaction ID of the current holder of that row lock.
  </para>

  <para>
   A speculative insertion lock consists of a transaction ID and a speculative
   insertion token. The speculative insertion token is displayed in the
   <structfield>objid</structfield> column.
  </para>

  <para>
   Advisory locks can be acquired on keys consisting of either a single
   <type>bigint</type> value or two integer values.
   A <type>bigint</type> key is displayed with its
   high-order half in the <structfield>classid</structfield> column, its low-order half
   in the <structfield>objid</structfield> column, and <structfield>objsubid</structfield> equal
   to 1. The original <type>bigint</type> value can be reassembled with the
   expression <literal>(classid::bigint &lt;&lt; 32) |
   objid::bigint</literal>. Integer keys are displayed with the
   first key in the
   <structfield>classid</structfield> column, the second key in the <structfield>objid</structfield>
   column, and <structfield>objsubid</structfield> equal to 2.  The actual meaning of
   the keys is up to the user.  Advisory locks are local to each database,
   so the <structfield>database</structfield> column is meaningful for an advisory lock.
  </para>

  <para>
   Apply transaction locks are used in parallel mode to apply the transaction
   in logical replication. The remote transaction ID is displayed in the
   <structfield>transactionid</structfield> column. The <structfield>objsubid</structfield>
   displays the lock subtype which is 0 for the lock used to synchronize the
   set of changes, and 1 for the lock used to wait for the transaction to
   finish to ensure commit order.
  </para>

  <para>
   <structname>pg_locks</structname> provides a global view of all locks
   in the database cluster, not only those relevant to the current database.
   Although its <structfield>relation</structfield> column can be joined
   against <link linkend="catalog-pg-class"><structname>pg_class</structname></link>.<structfield>oid</structfield> to identify locked
   relations, this will only work correctly for relations in the current
   database (those for which the <structfield>database</structfield> column
   is either the current database's OID or zero).
  </para>

  <para>
   The <structfield>pid</structfield> column can be joined to the
   <structfield>pid</structfield> column of the
   <link linkend="monitoring-pg-stat-activity-view">
   <structname>pg_stat_activity</structname></link>
   view to get more
   information on the session holding or awaiting each lock,
   for example
<programlisting>
SELECT * FROM pg_locks pl LEFT JOIN pg_stat_activity psa
    ON pl.pid = psa.pid;
</programlisting>
   Also, if you are using prepared transactions, the
   <structfield>virtualtransaction</structfield> column can be joined to the
   <structfield>transaction</structfield> column of the <link
   linkend="view-pg-prepared-xacts"><structname>pg_prepared_xacts</structname></link>
   view to get more information on prepared transactions that hold locks.
   (A prepared transaction can never be waiting for a lock,
   but it continues to hold the locks it acquired while running.)
   For example:
<programlisting>
SELECT * FROM pg_locks pl LEFT JOIN pg_prepared_xacts ppx
    ON pl.virtualtransaction = '-1/' || ppx.transaction;
</programlisting>
  </para>

  <para>
   While it is possible to obtain information about which processes block
   which other processes by joining <structname>pg_locks</structname> against
   itself, this is very difficult to get right in detail.  Such a query would
   have to encode knowledge about which lock modes conflict with which
   others.  Worse, the <structname>pg_locks</structname> view does not expose
   information about which processes are ahead of which others in lock wait
   queues, nor information about which processes are parallel workers running
   on behalf of which other client sessions.  It is better to use
   the <function>pg_blocking_pids()</function> function
   (see <xref linkend="functions-info-session-table"/>) to identify which
   process(es) a waiting process is blocked behind.
  </para>

  <para>
   The <structname>pg_locks</structname> view displays data from both the
   regular lock manager and the predicate lock manager, which are
   separate systems; in addition, the regular lock manager subdivides its
   locks into regular and <firstterm>fast-path</firstterm> locks.
   This data is not guaranteed to be entirely consistent.
   When the view is queried,
   data on fast-path locks (with <structfield>fastpath</structfield> = <literal>true</literal>)
   is gathered from each backend one at a time, without freezing the state of
   the entire lock manager, so it is possible for locks to be taken or
   released while information is gathered.  Note, however, that these locks are
   known not to conflict with any other lock currently in place.  After
   all backends have been queried for fast-path locks, the remainder of the
   regular lock manager is locked as a unit, and a consistent snapshot of all
   remaining locks is collected as an atomic action.  After unlocking the
   regular lock manager, the predicate lock manager is similarly locked and all
   predicate locks are collected as an atomic action.  Thus, with the exception
   of fast-path locks, each lock manager will deliver a consistent set of
   results, but as we do not lock both lock managers simultaneously, it is
   possible for locks to be taken or released after we interrogate the regular
   lock manager and before we interrogate the predicate lock manager.
  </para>

  <para>
   Locking the regular and/or predicate lock manager could have some
   impact on database performance if this view is very frequently accessed.
   The locks are held only for the minimum amount of time necessary to
   obtain data from the lock managers, but this does not completely eliminate
   the possibility of a performance impact.
  </para>

 </sect1>

 <sect1 id="view-pg-matviews">
  <title><structname>pg_matviews</structname></title>

  <indexterm zone="view-pg-matviews">
   <primary>pg_matviews</primary>
  </indexterm>

  <indexterm zone="view-pg-matviews">
   <primary>materialized views</primary>
  </indexterm>

  <para>
   The view <structname>pg_matviews</structname> provides access to
   useful information about each materialized view in the database.
  </para>

  <table>
   <title><structname>pg_matviews</structname> Columns</title>
   <tgroup cols="1">
    <thead>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       Column Type
      </para>
      <para>
       Description
      </para></entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>schemaname</structfield> <type>name</type>
       (references <link linkend="catalog-pg-namespace"><structname>pg_namespace</structname></link>.<structfield>nspname</structfield>)
      </para>
      <para>
       Name of schema containing materialized view
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>matviewname</structfield> <type>name</type>
       (references <link linkend="catalog-pg-class"><structname>pg_class</structname></link>.<structfield>relname</structfield>)
      </para>
      <para>
       Name of materialized view
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>matviewowner</structfield> <type>name</type>
       (references <link linkend="catalog-pg-authid"><structname>pg_authid</structname></link>.<structfield>rolname</structfield>)
      </para>
      <para>
       Name of materialized view's owner
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>tablespace</structfield> <type>name</type>
       (references <link linkend="catalog-pg-tablespace"><structname>pg_tablespace</structname></link>.<structfield>spcname</structfield>)
      </para>
      <para>
       Name of tablespace containing materialized view (null if default for database)
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>hasindexes</structfield> <type>bool</type>
      </para>
      <para>
       True if materialized view has (or recently had) any indexes
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>ispopulated</structfield> <type>bool</type>
      </para>
      <para>
       True if materialized view is currently populated
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>definition</structfield> <type>text</type>
      </para>
      <para>
       Materialized view definition (a reconstructed <xref linkend="sql-select"/> query)
      </para></entry>
     </row>
    </tbody>
   </tgroup>
  </table>

 </sect1>

 <sect1 id="view-pg-policies">
  <title><structname>pg_policies</structname></title>

  <indexterm zone="view-pg-policies">
   <primary>pg_policies</primary>
  </indexterm>

  <para>
   The view <structname>pg_policies</structname> provides access to
   useful information about each row-level security policy in the database.
  </para>

  <table>
   <title><structname>pg_policies</structname> Columns</title>
   <tgroup cols="1">
    <thead>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       Column Type
      </para>
      <para>
       Description
      </para></entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>schemaname</structfield> <type>name</type>
       (references <link linkend="catalog-pg-namespace"><structname>pg_namespace</structname></link>.<structfield>nspname</structfield>)
      </para>
      <para>
       Name of schema containing table policy is on
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>tablename</structfield> <type>name</type>
       (references <link linkend="catalog-pg-class"><structname>pg_class</structname></link>.<structfield>relname</structfield>)
      </para>
      <para>
       Name of table policy is on
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>policyname</structfield> <type>name</type>
       (references <link linkend="catalog-pg-policy"><structname>pg_policy</structname></link>.<structfield>polname</structfield>)
      </para>
      <para>
       Name of policy
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>permissive</structfield> <type>text</type>
      </para>
      <para>
       Is the policy permissive or restrictive?
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>roles</structfield> <type>name[]</type>
      </para>
      <para>
       The roles to which this policy applies
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>cmd</structfield> <type>text</type>
      </para>
      <para>
       The command type to which the policy is applied
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>qual</structfield> <type>text</type>
      </para>
      <para>
       The expression added to the security barrier qualifications for
       queries that this policy applies to
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>with_check</structfield> <type>text</type>
      </para>
      <para>
       The expression added to the WITH CHECK qualifications for
       queries that attempt to add rows to this table
      </para></entry>
     </row>
    </tbody>
   </tgroup>
  </table>

 </sect1>

 <sect1 id="view-pg-prepared-statements">
  <title><structname>pg_prepared_statements</structname></title>

  <indexterm zone="view-pg-prepared-statements">
   <primary>pg_prepared_statements</primary>
  </indexterm>

  <para>
   The <structname>pg_prepared_statements</structname> view displays
   all the prepared statements that are available in the current
   session. See <xref linkend="sql-prepare"/> for more information about prepared
   statements.
  </para>

  <para>
   <structname>pg_prepared_statements</structname> contains one row
   for each prepared statement. Rows are added to the view when a new
   prepared statement is created and removed when a prepared statement
   is released (for example, via the <link linkend="sql-deallocate"><command>DEALLOCATE</command></link> command).
  </para>

  <table>
   <title><structname>pg_prepared_statements</structname> Columns</title>
   <tgroup cols="1">
    <thead>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       Column Type
      </para>
      <para>
       Description
      </para></entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>name</structfield> <type>text</type>
      </para>
      <para>
       The identifier of the prepared statement
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>statement</structfield> <type>text</type>
      </para>
      <para>
       The query string submitted by the client to create this
       prepared statement. For prepared statements created via SQL,
       this is the <command>PREPARE</command> statement submitted by
       the client. For prepared statements created via the
       frontend/backend protocol, this is the text of the prepared
       statement itself.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>prepare_time</structfield> <type>timestamptz</type>
      </para>
      <para>
       The time at which the prepared statement was created
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>parameter_types</structfield> <type>regtype[]</type>
      </para>
      <para>
       The expected parameter types for the prepared statement in the
       form of an array of <type>regtype</type>. The OID corresponding
       to an element of this array can be obtained by casting the
       <type>regtype</type> value to <type>oid</type>.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>result_types</structfield> <type>regtype[]</type>
      </para>
      <para>
       The types of the columns returned by the prepared statement in the
       form of an array of <type>regtype</type>. The OID corresponding
       to an element of this array can be obtained by casting the
       <type>regtype</type> value to <type>oid</type>.
       If the prepared statement does not provide a result (e.g., a DML
       statement), then this field will be null.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>from_sql</structfield> <type>bool</type>
      </para>
      <para>
       <literal>true</literal> if the prepared statement was created
       via the <command>PREPARE</command> SQL command;
       <literal>false</literal> if the statement was prepared via the
       frontend/backend protocol
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>generic_plans</structfield> <type>int8</type>
      </para>
      <para>
       Number of times generic plan was chosen
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>custom_plans</structfield> <type>int8</type>
      </para>
      <para>
       Number of times custom plan was chosen
      </para></entry>
     </row>
    </tbody>
   </tgroup>
  </table>

  <para>
   The <structname>pg_prepared_statements</structname> view is read-only.
  </para>
 </sect1>

 <sect1 id="view-pg-prepared-xacts">
  <title><structname>pg_prepared_xacts</structname></title>

  <indexterm zone="view-pg-prepared-xacts">
   <primary>pg_prepared_xacts</primary>
  </indexterm>

  <para>
   The view <structname>pg_prepared_xacts</structname> displays
   information about transactions that are currently prepared for two-phase
   commit (see <xref linkend="sql-prepare-transaction"/> for details).
  </para>

  <para>
   <structname>pg_prepared_xacts</structname> contains one row per prepared
   transaction.  An entry is removed when the transaction is committed or
   rolled back.
  </para>

  <table>
   <title><structname>pg_prepared_xacts</structname> Columns</title>
   <tgroup cols="1">
    <thead>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       Column Type
      </para>
      <para>
       Description
      </para></entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>transaction</structfield> <type>xid</type>
      </para>
      <para>
       Numeric transaction identifier of the prepared transaction
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>gid</structfield> <type>text</type>
      </para>
      <para>
       Global transaction identifier that was assigned to the transaction
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>prepared</structfield> <type>timestamptz</type>
      </para>
      <para>
       Time at which the transaction was prepared for commit
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>owner</structfield> <type>name</type>
       (references <link linkend="catalog-pg-authid"><structname>pg_authid</structname></link>.<structfield>rolname</structfield>)
      </para>
      <para>
       Name of the user that executed the transaction
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>database</structfield> <type>name</type>
       (references <link linkend="catalog-pg-database"><structname>pg_database</structname></link>.<structfield>datname</structfield>)
      </para>
      <para>
       Name of the database in which the transaction was executed
      </para></entry>
     </row>
    </tbody>
   </tgroup>
  </table>

  <para>
   When the <structname>pg_prepared_xacts</structname> view is accessed, the
   internal transaction manager data structures are momentarily locked, and
   a copy is made for the view to display.  This ensures that the
   view produces a consistent set of results, while not blocking
   normal operations longer than necessary.  Nonetheless
   there could be some impact on database performance if this view is
   frequently accessed.
  </para>

 </sect1>

 <sect1 id="view-pg-publication-tables">
  <title><structname>pg_publication_tables</structname></title>

  <indexterm zone="view-pg-publication-tables">
   <primary>pg_publication_tables</primary>
  </indexterm>

  <para>
   The view <structname>pg_publication_tables</structname> provides
   information about the mapping between publications and information of
   tables they contain.  Unlike the underlying catalog
   <link linkend="catalog-pg-publication-rel"><structname>pg_publication_rel</structname></link>,
   this view expands publications defined as
   <link linkend="sql-createpublication-params-for-all-tables"><literal>FOR ALL TABLES</literal></link>
   and <link linkend="sql-createpublication-params-for-tables-in-schema"><literal>FOR TABLES IN SCHEMA</literal></link>,
   so for such publications there will be a row for each eligible table.
  </para>

  <table>
   <title><structname>pg_publication_tables</structname> Columns</title>
   <tgroup cols="1">
    <thead>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       Column Type
      </para>
      <para>
       Description
      </para></entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>pubname</structfield> <type>name</type>
       (references <link linkend="catalog-pg-publication"><structname>pg_publication</structname></link>.<structfield>pubname</structfield>)
      </para>
      <para>
       Name of publication
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>schemaname</structfield> <type>name</type>
       (references <link linkend="catalog-pg-namespace"><structname>pg_namespace</structname></link>.<structfield>nspname</structfield>)
      </para>
      <para>
       Name of schema containing table
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>tablename</structfield> <type>name</type>
       (references <link linkend="catalog-pg-class"><structname>pg_class</structname></link>.<structfield>relname</structfield>)
      </para>
      <para>
       Name of table
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>attnames</structfield> <type>name[]</type>
       (references <link linkend="catalog-pg-attribute"><structname>pg_attribute</structname></link>.<structfield>attname</structfield>)
      </para>
      <para>
       Names of table columns included in the publication. This contains all
       the columns of the table when the user didn't specify the column list
       for the table.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>rowfilter</structfield> <type>text</type>
      </para>
      <para>
       Expression for the table's publication qualifying condition
      </para></entry>
     </row>
    </tbody>
   </tgroup>
  </table>
 </sect1>

  <sect1 id="view-pg-replication-origin-status">
  <title><structname>pg_replication_origin_status</structname></title>

  <indexterm zone="view-pg-replication-origin-status">
   <primary>pg_replication_origin_status</primary>
  </indexterm>

  <para>
   The <structname>pg_replication_origin_status</structname> view
   contains information about how far replay for a certain origin has
   progressed.  For more on replication origins
   see <xref linkend="replication-origins"/>.
  </para>

  <table>
   <title><structname>pg_replication_origin_status</structname> Columns</title>
   <tgroup cols="1">
    <thead>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       Column Type
      </para>
      <para>
       Description
      </para></entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>local_id</structfield> <type>oid</type>
       (references <link linkend="catalog-pg-replication-origin"><structname>pg_replication_origin</structname></link>.<structfield>roident</structfield>)
      </para>
      <para>
       internal node identifier
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>external_id</structfield> <type>text</type>
       (references <link linkend="catalog-pg-replication-origin"><structname>pg_replication_origin</structname></link>.<structfield>roname</structfield>)
      </para>
      <para>
       external node identifier
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>remote_lsn</structfield> <type>pg_lsn</type>
      </para>
      <para>
       The origin node's LSN up to which data has been replicated.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>local_lsn</structfield> <type>pg_lsn</type>
      </para>
      <para>
       This node's LSN at which <literal>remote_lsn</literal> has
       been replicated. Used to flush commit records before persisting
       data to disk when using asynchronous commits.
      </para></entry>
     </row>
    </tbody>
   </tgroup>
  </table>
 </sect1>

 <sect1 id="view-pg-replication-slots">
  <title><structname>pg_replication_slots</structname></title>

  <indexterm zone="view-pg-replication-slots">
   <primary>pg_replication_slots</primary>
  </indexterm>

  <para>
   The <structname>pg_replication_slots</structname> view provides a listing
   of all replication slots that currently exist on the database cluster,
   along with their current state.
  </para>

  <para>
   For more on replication slots,
   see <xref linkend="streaming-replication-slots"/> and <xref linkend="logicaldecoding"/>.
  </para>

  <table>
   <title><structname>pg_replication_slots</structname> Columns</title>
   <tgroup cols="1">
    <thead>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       Column Type
      </para>
      <para>
       Description
      </para></entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>slot_name</structfield> <type>name</type>
      </para>
      <para>
       A unique, cluster-wide identifier for the replication slot
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>plugin</structfield> <type>name</type>
      </para>
      <para>
       The base name of the shared object containing the output plugin this logical slot is using, or null for physical slots.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>slot_type</structfield> <type>text</type>
      </para>
      <para>
       The slot type: <literal>physical</literal> or <literal>logical</literal>
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>datoid</structfield> <type>oid</type>
       (references <link linkend="catalog-pg-database"><structname>pg_database</structname></link>.<structfield>oid</structfield>)
      </para>
      <para>
       The OID of the database this slot is associated with, or
       null. Only logical slots have an associated database.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>database</structfield> <type>name</type>
       (references <link linkend="catalog-pg-database"><structname>pg_database</structname></link>.<structfield>datname</structfield>)
      </para>
      <para>
       The name of the database this slot is associated with, or
       null. Only logical slots have an associated database.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>temporary</structfield> <type>bool</type>
      </para>
      <para>
       True if this is a temporary replication slot. Temporary slots are
       not saved to disk and are automatically dropped on error or when
       the session has finished.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>active</structfield> <type>bool</type>
      </para>
      <para>
       True if this slot is currently actively being used
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>active_pid</structfield> <type>int4</type>
      </para>
      <para>
       The process ID of the session using this slot if the slot
       is currently actively being used. <literal>NULL</literal> if
       inactive.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>xmin</structfield> <type>xid</type>
      </para>
      <para>
       The oldest transaction that this slot needs the database to
       retain.  <literal>VACUUM</literal> cannot remove tuples deleted
       by any later transaction.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>catalog_xmin</structfield> <type>xid</type>
      </para>
      <para>
       The oldest transaction affecting the system catalogs that this
       slot needs the database to retain.  <literal>VACUUM</literal> cannot
       remove catalog tuples deleted by any later transaction.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>restart_lsn</structfield> <type>pg_lsn</type>
      </para>
      <para>
       The address (<literal>LSN</literal>) of oldest WAL which still
       might be required by the consumer of this slot and thus won't be
       automatically removed during checkpoints unless this LSN
       gets behind more than <xref linkend="guc-max-slot-wal-keep-size"/>
       from the current LSN.  <literal>NULL</literal>
       if the <literal>LSN</literal> of this slot has never been reserved.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>confirmed_flush_lsn</structfield> <type>pg_lsn</type>
      </para>
      <para>
       The address (<literal>LSN</literal>) up to which the logical
       slot's consumer has confirmed receiving data. Data corresponding to the
       transactions committed before this <literal>LSN</literal> is not
       available anymore. <literal>NULL</literal> for physical slots.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>wal_status</structfield> <type>text</type>
      </para>
      <para>
       Availability of WAL files claimed by this slot.
       Possible values are:
       <itemizedlist>
        <listitem>
         <para><literal>reserved</literal> means that the claimed files
          are within <varname>max_wal_size</varname>.</para>
        </listitem>
        <listitem>
         <para><literal>extended</literal> means
          that <varname>max_wal_size</varname> is exceeded but the files are
          still retained, either by the replication slot or
          by <varname>wal_keep_size</varname>.
         </para>
        </listitem>
        <listitem>
         <para>
          <literal>unreserved</literal> means that the slot no longer
          retains the required WAL files and some of them are to be removed at
          the next checkpoint.  This state can return
          to <literal>reserved</literal> or <literal>extended</literal>.
         </para>
        </listitem>
        <listitem>
         <para>
          <literal>lost</literal> means that some required WAL files have
          been removed and this slot is no longer usable.
         </para>
        </listitem>
       </itemizedlist>
       The last two states are seen only when
       <xref linkend="guc-max-slot-wal-keep-size"/> is
       non-negative. If <structfield>restart_lsn</structfield> is NULL, this
       field is null.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>safe_wal_size</structfield> <type>int8</type>
      </para>
      <para>
       The number of bytes that can be written to WAL such that this slot
       is not in danger of getting in state "lost".  It is NULL for lost
       slots, as well as if <varname>max_slot_wal_keep_size</varname>
       is <literal>-1</literal>.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>two_phase</structfield> <type>bool</type>
      </para>
      <para>
       True if the slot is enabled for decoding prepared transactions.  Always
       false for physical slots.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>inactive_since</structfield> <type>timestamptz</type>
      </para>
      <para>
        The time since the slot has become inactive.
        <literal>NULL</literal> if the slot is currently being used.
        Note that for slots on the standby that are being synced from a
        primary server (whose <structfield>synced</structfield> field is
        <literal>true</literal>), the
        <structfield>inactive_since</structfield> indicates the last
        synchronization (see
        <xref linkend="logicaldecoding-replication-slots-synchronization"/>)
        time.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>conflicting</structfield> <type>bool</type>
      </para>
      <para>
       True if this logical slot conflicted with recovery (and so is now
       invalidated). When this column is true, check
       <structfield>invalidation_reason</structfield> column for the conflict
       reason. Always NULL for physical slots.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>invalidation_reason</structfield> <type>text</type>
      </para>
      <para>
       The reason for the slot's invalidation. It is set for both logical and
       physical slots. <literal>NULL</literal> if the slot is not invalidated.
       Possible values are:
       <itemizedlist spacing="compact">
        <listitem>
         <para>
          <literal>wal_removed</literal> means that the required WAL has been
          removed.
         </para>
        </listitem>
        <listitem>
         <para>
          <literal>rows_removed</literal> means that the required rows have
          been removed. It is set only for logical slots.
         </para>
        </listitem>
        <listitem>
         <para>
          <literal>wal_level_insufficient</literal> means that the
          primary doesn't have a <xref linkend="guc-wal-level"/> sufficient to
          perform logical decoding.  It is set only for logical slots.
         </para>
        </listitem>
       </itemizedlist>
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>failover</structfield> <type>bool</type>
      </para>
      <para>
       True if this is a logical slot enabled to be synced to the standbys
       so that logical replication can be resumed from the new primary
       after failover. Always false for physical slots.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>synced</structfield> <type>bool</type>
      </para>
      <para>
       True if this is a logical slot that was synced from a primary server.
       On a hot standby, the slots with the synced column marked as true can
       neither be used for logical decoding nor dropped manually. The value
       of this column has no meaning on the primary server; the column value on
       the primary is default false for all slots but may (if leftover from a
       promoted standby) also be true.
      </para></entry>
     </row>

    </tbody>
   </tgroup>
  </table>
 </sect1>

 <sect1 id="view-pg-roles">
  <title><structname>pg_roles</structname></title>

  <indexterm zone="view-pg-roles">
   <primary>pg_roles</primary>
  </indexterm>

  <para>
   The view <structname>pg_roles</structname> provides access to
   information about database roles.  This is simply a publicly
   readable view of
   <link linkend="catalog-pg-authid"><structname>pg_authid</structname></link>
   that blanks out the password field.
  </para>

  <table>
   <title><structname>pg_roles</structname> Columns</title>
   <tgroup cols="1">
    <thead>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       Column Type
      </para>
      <para>
       Description
      </para></entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>rolname</structfield> <type>name</type>
      </para>
      <para>
       Role name
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>rolsuper</structfield> <type>bool</type>
      </para>
      <para>
       Role has superuser privileges
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>rolinherit</structfield> <type>bool</type>
      </para>
      <para>
       Role automatically inherits privileges of roles it is a
       member of
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>rolcreaterole</structfield> <type>bool</type>
      </para>
      <para>
       Role can create more roles
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>rolcreatedb</structfield> <type>bool</type>
      </para>
      <para>
       Role can create databases
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>rolcanlogin</structfield> <type>bool</type>
      </para>
      <para>
       Role can log in. That is, this role can be given as the initial
       session authorization identifier
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>rolreplication</structfield> <type>bool</type>
      </para>
      <para>
       Role is a replication role. A replication role can initiate replication
       connections and create and drop replication slots.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>rolconnlimit</structfield> <type>int4</type>
      </para>
      <para>
       For roles that can log in, this sets maximum number of concurrent
       connections this role can make.  -1 means no limit.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>rolpassword</structfield> <type>text</type>
      </para>
      <para>
       Not the password (always reads as <literal>********</literal>)
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>rolvaliduntil</structfield> <type>timestamptz</type>
      </para>
      <para>
       Password expiry time (only used for password authentication);
       null if no expiration
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>rolbypassrls</structfield> <type>bool</type>
      </para>
      <para>
       Role bypasses every row-level security policy, see
       <xref linkend="ddl-rowsecurity"/> for more information.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>rolconfig</structfield> <type>text[]</type>
      </para>
      <para>
       Role-specific defaults for run-time configuration variables
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>oid</structfield> <type>oid</type>
       (references <link linkend="catalog-pg-authid"><structname>pg_authid</structname></link>.<structfield>oid</structfield>)
      </para>
      <para>
       ID of role
      </para></entry>
     </row>
    </tbody>
   </tgroup>
  </table>

 </sect1>

 <sect1 id="view-pg-rules">
  <title><structname>pg_rules</structname></title>

  <indexterm zone="view-pg-rules">
   <primary>pg_rules</primary>
  </indexterm>

  <para>
   The view <structname>pg_rules</structname> provides access to
   useful information about query rewrite rules.
  </para>

  <table>
   <title><structname>pg_rules</structname> Columns</title>
   <tgroup cols="1">
    <thead>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       Column Type
      </para>
      <para>
       Description
      </para></entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>schemaname</structfield> <type>name</type>
       (references <link linkend="catalog-pg-namespace"><structname>pg_namespace</structname></link>.<structfield>nspname</structfield>)
      </para>
      <para>
       Name of schema containing table
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>tablename</structfield> <type>name</type>
       (references <link linkend="catalog-pg-class"><structname>pg_class</structname></link>.<structfield>relname</structfield>)
      </para>
      <para>
       Name of table the rule is for
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>rulename</structfield> <type>name</type>
       (references <link linkend="catalog-pg-rewrite"><structname>pg_rewrite</structname></link>.<structfield>rulename</structfield>)
      </para>
      <para>
       Name of rule
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>definition</structfield> <type>text</type>
      </para>
      <para>
       Rule definition (a reconstructed creation command)
      </para></entry>
     </row>
    </tbody>
   </tgroup>
  </table>

  <para>
   The <structname>pg_rules</structname> view excludes the <literal>ON SELECT</literal> rules
   of views and materialized views; those can be seen in
   <link linkend="view-pg-views"><structname>pg_views</structname></link> and <link linkend="view-pg-matviews"><structname>pg_matviews</structname></link>.
  </para>

 </sect1>

 <sect1 id="view-pg-seclabels">
  <title><structname>pg_seclabels</structname></title>

  <indexterm zone="view-pg-seclabels">
   <primary>pg_seclabels</primary>
  </indexterm>

  <para>
   The view <structname>pg_seclabels</structname> provides information about
   security labels.  It as an easier-to-query version of the
   <link linkend="catalog-pg-seclabel"><structname>pg_seclabel</structname></link> catalog.
  </para>

  <table>
   <title><structname>pg_seclabels</structname> Columns</title>
   <tgroup cols="1">
    <thead>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       Column Type
      </para>
      <para>
       Description
      </para></entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>objoid</structfield> <type>oid</type>
       (references any OID column)
      </para>
      <para>
       The OID of the object this security label pertains to
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>classoid</structfield> <type>oid</type>
       (references <link linkend="catalog-pg-class"><structname>pg_class</structname></link>.<structfield>oid</structfield>)
      </para>
      <para>
       The OID of the system catalog this object appears in
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>objsubid</structfield> <type>int4</type>
      </para>
      <para>
       For a security label on a table column, this is the column number (the
       <structfield>objoid</structfield> and <structfield>classoid</structfield> refer to
       the table itself).  For all other object types, this column is
       zero.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>objtype</structfield> <type>text</type>
      </para>
      <para>
       The type of object to which this label applies, as text.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>objnamespace</structfield> <type>oid</type>
       (references <link linkend="catalog-pg-namespace"><structname>pg_namespace</structname></link>.<structfield>oid</structfield>)
      </para>
      <para>
       The OID of the namespace for this object, if applicable;
       otherwise NULL.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>objname</structfield> <type>text</type>
      </para>
      <para>
       The name of the object to which this label applies, as text.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>provider</structfield> <type>text</type>
       (references <link linkend="catalog-pg-seclabel"><structname>pg_seclabel</structname></link>.<structfield>provider</structfield>)
      </para>
      <para>
       The label provider associated with this label.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>label</structfield> <type>text</type>
       (references <link linkend="catalog-pg-seclabel"><structname>pg_seclabel</structname></link>.<structfield>label</structfield>)
      </para>
      <para>
       The security label applied to this object.
      </para></entry>
     </row>
    </tbody>
   </tgroup>
  </table>
 </sect1>

 <sect1 id="view-pg-sequences">
  <title><structname>pg_sequences</structname></title>

  <indexterm zone="view-pg-sequences">
   <primary>pg_sequences</primary>
  </indexterm>

  <para>
   The view <structname>pg_sequences</structname> provides access to
   useful information about each sequence in the database.
  </para>

  <table>
   <title><structname>pg_sequences</structname> Columns</title>
   <tgroup cols="1">
    <thead>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       Column Type
      </para>
      <para>
       Description
      </para></entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>schemaname</structfield> <type>name</type>
       (references <link linkend="catalog-pg-namespace"><structname>pg_namespace</structname></link>.<structfield>nspname</structfield>)
      </para>
      <para>
       Name of schema containing sequence
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>sequencename</structfield> <type>name</type>
       (references <link linkend="catalog-pg-class"><structname>pg_class</structname></link>.<structfield>relname</structfield>)
      </para>
      <para>
       Name of sequence
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>sequenceowner</structfield> <type>name</type>
       (references <link linkend="catalog-pg-authid"><structname>pg_authid</structname></link>.<structfield>rolname</structfield>)
      </para>
      <para>
       Name of sequence's owner
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>data_type</structfield> <type>regtype</type>
       (references <link linkend="catalog-pg-type"><structname>pg_type</structname></link>.<structfield>oid</structfield>)
      </para>
      <para>
       Data type of the sequence
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>start_value</structfield> <type>int8</type>
      </para>
      <para>
       Start value of the sequence
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>min_value</structfield> <type>int8</type>
      </para>
      <para>
       Minimum value of the sequence
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>max_value</structfield> <type>int8</type>
      </para>
      <para>
       Maximum value of the sequence
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>increment_by</structfield> <type>int8</type>
      </para>
      <para>
       Increment value of the sequence
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>cycle</structfield> <type>bool</type>
      </para>
      <para>
       Whether the sequence cycles
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>cache_size</structfield> <type>int8</type>
      </para>
      <para>
       Cache size of the sequence
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>last_value</structfield> <type>int8</type>
      </para>
      <para>
       The last sequence value written to disk.  If caching is used,
       this value can be greater than the last value handed out from the
       sequence.
      </para></entry>
     </row>
    </tbody>
   </tgroup>
  </table>

  <para>
   The <structfield>last_value</structfield> column will read as null if any of
   the following are true:
   <itemizedlist>
    <listitem>
     <para>
      The sequence has not been read from yet.
     </para>
    </listitem>
    <listitem>
     <para>
      The current user does not have <literal>USAGE</literal> or
      <literal>SELECT</literal> privilege on the sequence.
     </para>
    </listitem>
    <listitem>
     <para>
      The sequence is unlogged and the server is a standby.
     </para>
    </listitem>
   </itemizedlist>
  </para>

 </sect1>

 <sect1 id="view-pg-settings">
  <title><structname>pg_settings</structname></title>

  <indexterm zone="view-pg-settings">
   <primary>pg_settings</primary>
  </indexterm>

  <para>
   The view <structname>pg_settings</structname> provides access to
   run-time parameters of the server.  It is essentially an alternative
   interface to the <link linkend="sql-show"><command>SHOW</command></link>
   and <link linkend="sql-set"><command>SET</command></link> commands.
   It also provides access to some facts about each parameter that are
   not directly available from <link linkend="sql-show"><command>SHOW</command></link>, such as minimum and
   maximum values.
  </para>

  <table>
   <title><structname>pg_settings</structname> Columns</title>
   <tgroup cols="1">
    <thead>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       Column Type
      </para>
      <para>
       Description
      </para></entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>name</structfield> <type>text</type>
      </para>
      <para>
       Run-time configuration parameter name
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>setting</structfield> <type>text</type>
      </para>
      <para>
       Current value of the parameter
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>unit</structfield> <type>text</type>
      </para>
      <para>
       Implicit unit of the parameter
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>category</structfield> <type>text</type>
      </para>
      <para>
       Logical group of the parameter
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>short_desc</structfield> <type>text</type>
      </para>
      <para>
       A brief description of the parameter
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>extra_desc</structfield> <type>text</type>
      </para>
      <para>
       Additional, more detailed, description of the parameter
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>context</structfield> <type>text</type>
      </para>
      <para>
       Context required to set the parameter's value (see below)
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>vartype</structfield> <type>text</type>
      </para>
      <para>
       Parameter type (<literal>bool</literal>, <literal>enum</literal>,
       <literal>integer</literal>, <literal>real</literal>, or <literal>string</literal>)
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>source</structfield> <type>text</type>
      </para>
      <para>
       Source of the current parameter value
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>min_val</structfield> <type>text</type>
      </para>
      <para>
       Minimum allowed value of the parameter (null for non-numeric
       values)
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>max_val</structfield> <type>text</type>
      </para>
      <para>
       Maximum allowed value of the parameter (null for non-numeric
       values)
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>enumvals</structfield> <type>text[]</type>
      </para>
      <para>
       Allowed values of an enum parameter (null for non-enum
       values)
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>boot_val</structfield> <type>text</type>
      </para>
      <para>
       Parameter value assumed at server startup if the parameter is
       not otherwise set
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>reset_val</structfield> <type>text</type>
      </para>
      <para>
       Value that <link linkend="sql-reset"><command>RESET</command></link> would reset the parameter to
       in the current session
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>sourcefile</structfield> <type>text</type>
      </para>
      <para>
       Configuration file the current value was set in (null for
       values set from sources other than configuration files, or when
       examined by a user who neither is a superuser nor has privileges of
       <literal>pg_read_all_settings</literal>); helpful when using
       <literal>include</literal> directives in configuration files
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>sourceline</structfield> <type>int4</type>
      </para>
      <para>
       Line number within the configuration file the current value was
       set at (null for values set from sources other than configuration files,
       or when examined by a user who neither is a superuser nor has privileges of
       <literal>pg_read_all_settings</literal>).
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>pending_restart</structfield> <type>bool</type>
      </para>
      <para>
       <literal>true</literal> if the value has been changed in the
       configuration file but needs a restart; or <literal>false</literal>
       otherwise.
      </para></entry>
     </row>
    </tbody>
   </tgroup>
  </table>

  <para>
   There are several possible values of <structfield>context</structfield>.
   In order of decreasing difficulty of changing the setting, they are:
  </para>

  <variablelist>
   <varlistentry>
    <!-- PGC_INTERNAL -->
    <term><literal>internal</literal></term>
    <listitem>
     <para>
      These settings cannot be changed directly; they reflect internally
      determined values.  Some of them may be adjustable by rebuilding the
      server with different configuration options, or by changing options
      supplied to <application>initdb</application>.
     </para>
    </listitem>
   </varlistentry>
   <varlistentry>
    <!-- PGC_POSTMASTER -->
    <term><literal>postmaster</literal></term>
    <listitem>
     <para>
      These settings can only be applied when the server starts, so any change
      requires restarting the server.  Values for these settings are typically
      stored in the <filename>postgresql.conf</filename> file, or passed on
      the command line when starting the server.  Of course, settings with any
      of the lower <structfield>context</structfield> types can also be
      set at server start time.
     </para>
    </listitem>
   </varlistentry>
   <varlistentry>
    <!-- PGC_SIGHUP -->
    <term><literal>sighup</literal></term>
    <listitem>
     <para>
      Changes to these settings can be made in
      <filename>postgresql.conf</filename> without restarting the server.
      Send a <systemitem>SIGHUP</systemitem> signal to the postmaster to
      cause it to re-read <filename>postgresql.conf</filename> and apply
      the changes.  The postmaster will also forward the
      <systemitem>SIGHUP</systemitem> signal to its child processes so that
      they all pick up the new value.
     </para>
    </listitem>
   </varlistentry>
   <varlistentry>
    <!-- PGC_SU_BACKEND -->
    <term><literal>superuser-backend</literal></term>
    <listitem>
     <para>
      Changes to these settings can be made in
      <filename>postgresql.conf</filename> without restarting the server.
      They can also be set for a particular session in the connection request
      packet (for example, via <application>libpq</application>'s <literal>PGOPTIONS</literal>
      environment variable), but only if the connecting user is a superuser
      or has been granted the appropriate <literal>SET</literal> privilege.
      However, these settings never change in a session after it is started.
      If you change them in <filename>postgresql.conf</filename>, send a
      <systemitem>SIGHUP</systemitem> signal to the postmaster to cause it to
      re-read <filename>postgresql.conf</filename>.  The new values will only
      affect subsequently-launched sessions.
     </para>
    </listitem>
   </varlistentry>
   <varlistentry>
    <!-- PGC_BACKEND -->
    <term><literal>backend</literal></term>
    <listitem>
     <para>
      Changes to these settings can be made in
      <filename>postgresql.conf</filename> without restarting the server.
      They can also be set for a particular session in the connection request
      packet (for example, via <application>libpq</application>'s <literal>PGOPTIONS</literal>
      environment variable); any user can make such a change for their session.
      However, these settings never change in a session after it is started.
      If you change them in <filename>postgresql.conf</filename>, send a
      <systemitem>SIGHUP</systemitem> signal to the postmaster to cause it to
      re-read <filename>postgresql.conf</filename>.  The new values will only
      affect subsequently-launched sessions.
     </para>
    </listitem>
   </varlistentry>
   <varlistentry>
    <!-- PGC_SUSET -->
    <term><literal>superuser</literal></term>
    <listitem>
     <para>
      These settings can be set from <filename>postgresql.conf</filename>,
      or within a session via the <command>SET</command> command; but only superusers
      and users with the appropriate <literal>SET</literal> privilege
      can change them via <command>SET</command>.  Changes in
      <filename>postgresql.conf</filename> will affect existing sessions
      only if no session-local value has been established with <command>SET</command>.
     </para>
    </listitem>
   </varlistentry>
   <varlistentry>
    <!-- PGC_USERSET -->
    <term><literal>user</literal></term>
    <listitem>
     <para>
      These settings can be set from <filename>postgresql.conf</filename>,
      or within a session via the <command>SET</command> command.  Any user is
      allowed to change their session-local value.  Changes in
      <filename>postgresql.conf</filename> will affect existing sessions
      only if no session-local value has been established with <command>SET</command>.
     </para>
    </listitem>
   </varlistentry>
  </variablelist>

  <para>
   See <xref linkend="config-setting"/> for more information about the various
   ways to change these parameters.
  </para>

  <para>
   This view cannot be inserted into or deleted from, but it can be updated.  An
   <command>UPDATE</command> applied to a row of <structname>pg_settings</structname>
   is equivalent to executing the <command>SET</command> command on that named
   parameter. The change only affects the value used by the current
   session. If an <command>UPDATE</command> is issued within a transaction
   that is later aborted, the effects of the <command>UPDATE</command> command
   disappear when the transaction is rolled back. Once the surrounding
   transaction is committed, the effects will persist until the end of the
   session, unless overridden by another <command>UPDATE</command> or
   <command>SET</command>.
  </para>

  <para>
   This view does not
   display <link linkend="runtime-config-custom">customized options</link>
   unless the extension module that defines them has been loaded by the
   backend process executing the query (e.g., via a mention in
   <xref linkend="guc-shared-preload-libraries"/>,
   a call to a C function in the extension, or the
   <link linkend="sql-load"><command>LOAD</command></link> command).
   For example, since <link linkend="archive-modules">archive modules</link>
   are normally loaded only by the archiver process not regular sessions,
   this view will not display any customized options defined by such modules
   unless special action is taken to load them into the backend process
   executing the query.
  </para>

 </sect1>

 <sect1 id="view-pg-shadow">
  <title><structname>pg_shadow</structname></title>

  <indexterm zone="view-pg-shadow">
   <primary>pg_shadow</primary>
  </indexterm>

  <para>
   The view <structname>pg_shadow</structname> exists for backwards
   compatibility: it emulates a catalog that existed in
   <productname>PostgreSQL</productname> before version 8.1.
   It shows properties of all roles that are marked as
   <structfield>rolcanlogin</structfield> in
   <link linkend="catalog-pg-authid"><structname>pg_authid</structname></link>.
  </para>

  <para>
   The name stems from the fact that this table
   should not be readable by the public since it contains passwords.
   <link linkend="view-pg-user"><structname>pg_user</structname></link>
   is a publicly readable view on
   <structname>pg_shadow</structname> that blanks out the password field.
  </para>

  <table>
   <title><structname>pg_shadow</structname> Columns</title>
   <tgroup cols="1">
    <thead>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       Column Type
      </para>
      <para>
       Description
      </para></entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>usename</structfield> <type>name</type>
       (references <link linkend="catalog-pg-authid"><structname>pg_authid</structname></link>.<structfield>rolname</structfield>)
      </para>
      <para>
       User name
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>usesysid</structfield> <type>oid</type>
       (references <link linkend="catalog-pg-authid"><structname>pg_authid</structname></link>.<structfield>oid</structfield>)
      </para>
      <para>
       ID of this user
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>usecreatedb</structfield> <type>bool</type>
      </para>
      <para>
       User can create databases
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>usesuper</structfield> <type>bool</type>
      </para>
      <para>
       User is a superuser
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>userepl</structfield> <type>bool</type>
      </para>
      <para>
       User can initiate streaming replication and put the system in and
       out of backup mode.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>usebypassrls</structfield> <type>bool</type>
      </para>
      <para>
       User bypasses every row-level security policy, see
       <xref linkend="ddl-rowsecurity"/> for more information.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>passwd</structfield> <type>text</type>
      </para>
      <para>
       Password (possibly encrypted); null if none.  See
       <link linkend="catalog-pg-authid"><structname>pg_authid</structname></link>
       for details of how encrypted passwords are stored.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>valuntil</structfield> <type>timestamptz</type>
      </para>
      <para>
       Password expiry time (only used for password authentication)
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>useconfig</structfield> <type>text[]</type>
      </para>
      <para>
       Session defaults for run-time configuration variables
      </para></entry>
     </row>
    </tbody>
   </tgroup>
  </table>

 </sect1>

 <sect1 id="view-pg-shmem-allocations">
  <title><structname>pg_shmem_allocations</structname></title>

  <indexterm zone="view-pg-shmem-allocations">
   <primary>pg_shmem_allocations</primary>
  </indexterm>

  <para>
   The <structname>pg_shmem_allocations</structname> view shows allocations
   made from the server's main shared memory segment.  This includes both
   memory allocated by <productname>PostgreSQL</productname> itself and memory
   allocated by extensions using the mechanisms detailed in
   <xref linkend="xfunc-shared-addin" />.
  </para>

  <para>
   Note that this view does not include memory allocated using the dynamic
   shared memory infrastructure.
  </para>

  <table>
   <title><structname>pg_shmem_allocations</structname> Columns</title>
   <tgroup cols="1">
    <thead>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       Column Type
      </para>
      <para>
       Description
      </para></entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>name</structfield> <type>text</type>
      </para>
      <para>
       The name of the shared memory allocation. NULL for unused memory
       and <literal>&lt;anonymous&gt;</literal> for anonymous
       allocations.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>off</structfield> <type>int8</type>
      </para>
      <para>
       The offset at which the allocation starts. NULL for anonymous
       allocations, since details related to them are not known.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>size</structfield> <type>int8</type>
      </para>
      <para>
       Size of the allocation in bytes
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>allocated_size</structfield> <type>int8</type>
      </para>
      <para>
       Size of the allocation in bytes including padding. For anonymous
       allocations, no information about padding is available, so the
       <literal>size</literal> and <literal>allocated_size</literal> columns
       will always be equal. Padding is not meaningful for free memory, so
       the columns will be equal in that case also.
      </para></entry>
     </row>
    </tbody>
   </tgroup>
  </table>

  <para>
   Anonymous allocations are allocations that have been made
   with <literal>ShmemAlloc()</literal> directly, rather than via
   <literal>ShmemInitStruct()</literal> or
   <literal>ShmemInitHash()</literal>.
  </para>

  <para>
   By default, the <structname>pg_shmem_allocations</structname> view can be
   read only by superusers or roles with privileges of the
   <literal>pg_read_all_stats</literal> role.
  </para>
 </sect1>

 <sect1 id="view-pg-stats">
  <title><structname>pg_stats</structname></title>

  <indexterm zone="view-pg-stats">
   <primary>pg_stats</primary>
  </indexterm>

  <para>
   The view <structname>pg_stats</structname> provides access to
   the information stored in the <link
   linkend="catalog-pg-statistic"><structname>pg_statistic</structname></link>
   catalog.  This view allows access only to rows of
   <link linkend="catalog-pg-statistic"><structname>pg_statistic</structname></link> that correspond to tables the
   user has permission to read, and therefore it is safe to allow public
   read access to this view.
  </para>

  <para>
   <structname>pg_stats</structname> is also designed to present the
   information in a more readable format than the underlying catalog
   &mdash; at the cost that its schema must be extended whenever new slot types
   are defined for <link linkend="catalog-pg-statistic"><structname>pg_statistic</structname></link>.
  </para>

  <table>
   <title><structname>pg_stats</structname> Columns</title>
   <tgroup cols="1">
    <thead>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       Column Type
      </para>
      <para>
       Description
      </para></entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>schemaname</structfield> <type>name</type>
       (references <link linkend="catalog-pg-namespace"><structname>pg_namespace</structname></link>.<structfield>nspname</structfield>)
      </para>
      <para>
       Name of schema containing table
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>tablename</structfield> <type>name</type>
       (references <link linkend="catalog-pg-class"><structname>pg_class</structname></link>.<structfield>relname</structfield>)
      </para>
      <para>
       Name of table
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>attname</structfield> <type>name</type>
       (references <link linkend="catalog-pg-attribute"><structname>pg_attribute</structname></link>.<structfield>attname</structfield>)
      </para>
      <para>
       Name of column described by this row
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>inherited</structfield> <type>bool</type>
      </para>
      <para>
       If true, this row includes values from child tables, not just the
       values in the specified table
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>null_frac</structfield> <type>float4</type>
      </para>
      <para>
       Fraction of column entries that are null
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>avg_width</structfield> <type>int4</type>
      </para>
      <para>
       Average width in bytes of column's entries
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>n_distinct</structfield> <type>float4</type>
      </para>
      <para>
       If greater than zero, the estimated number of distinct values in the
       column.  If less than zero, the negative of the number of distinct
       values divided by the number of rows.  (The negated form is used when
       <command>ANALYZE</command> believes that the number of distinct values is
       likely to increase as the table grows; the positive form is used when
       the column seems to have a fixed number of possible values.)  For
       example, -1 indicates a unique column in which the number of distinct
       values is the same as the number of rows.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>most_common_vals</structfield> <type>anyarray</type>
      </para>
      <para>
       A list of the most common values in the column. (Null if
       no values seem to be more common than any others.)
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>most_common_freqs</structfield> <type>float4[]</type>
      </para>
      <para>
       A list of the frequencies of the most common values,
       i.e., number of occurrences of each divided by total number of rows.
       (Null when <structfield>most_common_vals</structfield> is.)
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>histogram_bounds</structfield> <type>anyarray</type>
      </para>
      <para>
       A list of values that divide the column's values into groups of
       approximately equal population.  The values in
       <structfield>most_common_vals</structfield>, if present, are omitted from this
       histogram calculation.  (This column is null if the column data type
       does not have a <literal>&lt;</literal> operator or if the
       <structfield>most_common_vals</structfield> list accounts for the entire
       population.)
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>correlation</structfield> <type>float4</type>
      </para>
      <para>
       Statistical correlation between physical row ordering and
       logical ordering of the column values.  This ranges from -1 to +1.
       When the value is near -1 or +1, an index scan on the column will
       be estimated to be cheaper than when it is near zero, due to reduction
       of random access to the disk.  (This column is null if the column data
       type does not have a <literal>&lt;</literal> operator.)
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>most_common_elems</structfield> <type>anyarray</type>
      </para>
      <para>
       A list of non-null element values most often appearing within values of
       the column. (Null for scalar types.)
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>most_common_elem_freqs</structfield> <type>float4[]</type>
      </para>
      <para>
       A list of the frequencies of the most common element values, i.e., the
       fraction of rows containing at least one instance of the given value.
       Two or three additional values follow the per-element frequencies;
       these are the minimum and maximum of the preceding per-element
       frequencies, and optionally the frequency of null elements.
       (Null when <structfield>most_common_elems</structfield> is.)
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>elem_count_histogram</structfield> <type>float4[]</type>
      </para>
      <para>
       A histogram of the counts of distinct non-null element values within the
       values of the column, followed by the average number of distinct
       non-null elements.  (Null for scalar types.)
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>range_length_histogram</structfield> <type>anyarray</type>
      </para>
      <para>
       A histogram of the lengths of non-empty and non-null range values of a
       range type column. (Null for non-range types.)
      </para>
      <para>
       This histogram is calculated using the <literal>subtype_diff</literal>
       range function regardless of whether range bounds are inclusive.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>range_empty_frac</structfield> <type>float4</type>
      </para>
      <para>
       Fraction of column entries whose values are empty ranges.
       (Null for non-range types.)
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>range_bounds_histogram</structfield> <type>anyarray</type>
      </para>
      <para>
       A histogram of lower and upper bounds of non-empty and non-null range
       values. (Null for non-range types.)
      </para>
      <para>
       These two histograms are represented as a single array of ranges, whose
       lower bounds represent the histogram of lower bounds, and upper bounds
       represent the histogram of upper bounds.
      </para></entry>
     </row>
    </tbody>
   </tgroup>
  </table>

  <para>
   The maximum number of entries in the array fields can be controlled on a
   column-by-column basis using the <link linkend="sql-altertable"><command>ALTER
   TABLE SET STATISTICS</command></link>
   command, or globally by setting the
   <xref linkend="guc-default-statistics-target"/> run-time parameter.
  </para>

 </sect1>

 <sect1 id="view-pg-stats-ext">
  <title><structname>pg_stats_ext</structname></title>

  <indexterm zone="view-pg-stats-ext">
   <primary>pg_stats_ext</primary>
  </indexterm>

  <para>
   The view <structname>pg_stats_ext</structname> provides access to
   information about each extended statistics object in the database,
   combining information stored in the <link
   linkend="catalog-pg-statistic-ext"><structname>pg_statistic_ext</structname></link>
   and <link linkend="catalog-pg-statistic-ext-data"><structname>pg_statistic_ext_data</structname></link>
   catalogs.  This view allows access only to rows of
   <link linkend="catalog-pg-statistic-ext"><structname>pg_statistic_ext</structname></link> and <link linkend="catalog-pg-statistic-ext-data"><structname>pg_statistic_ext_data</structname></link>
   that correspond to tables the user owns, and therefore
   it is safe to allow public read access to this view.
  </para>

  <para>
   <structname>pg_stats_ext</structname> is also designed to present the
   information in a more readable format than the underlying catalogs
   &mdash; at the cost that its schema must be extended whenever new types
   of extended statistics are added to <link linkend="catalog-pg-statistic-ext"><structname>pg_statistic_ext</structname></link>.
  </para>

  <table>
   <title><structname>pg_stats_ext</structname> Columns</title>
   <tgroup cols="1">
    <thead>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       Column Type
      </para>
      <para>
       Description
      </para></entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>schemaname</structfield> <type>name</type>
       (references <link linkend="catalog-pg-namespace"><structname>pg_namespace</structname></link>.<structfield>nspname</structfield>)
      </para>
      <para>
       Name of schema containing table
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>tablename</structfield> <type>name</type>
       (references <link linkend="catalog-pg-class"><structname>pg_class</structname></link>.<structfield>relname</structfield>)
      </para>
      <para>
       Name of table
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>statistics_schemaname</structfield> <type>name</type>
       (references <link linkend="catalog-pg-namespace"><structname>pg_namespace</structname></link>.<structfield>nspname</structfield>)
      </para>
      <para>
       Name of schema containing extended statistics object
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>statistics_name</structfield> <type>name</type>
       (references <link linkend="catalog-pg-statistic-ext"><structname>pg_statistic_ext</structname></link>.<structfield>stxname</structfield>)
      </para>
      <para>
       Name of extended statistics object
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>statistics_owner</structfield> <type>name</type>
       (references <link linkend="catalog-pg-authid"><structname>pg_authid</structname></link>.<structfield>rolname</structfield>)
      </para>
      <para>
       Owner of the extended statistics object
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>attnames</structfield> <type>name[]</type>
       (references <link linkend="catalog-pg-attribute"><structname>pg_attribute</structname></link>.<structfield>attname</structfield>)
      </para>
      <para>
       Names of the columns included in the extended statistics object
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>exprs</structfield> <type>text[]</type>
      </para>
      <para>
       Expressions included in the extended statistics object
      </para></entry>
      </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>kinds</structfield> <type>char[]</type>
      </para>
      <para>
       Types of extended statistics object enabled for this record
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>inherited</structfield> <type>bool</type>
       (references <link linkend="catalog-pg-statistic-ext-data"><structname>pg_statistic_ext_data</structname></link>.<structfield>stxdinherit</structfield>)
      </para>
      <para>
       If true, the stats include values from child tables, not just the
       values in the specified relation
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>n_distinct</structfield> <type>pg_ndistinct</type>
      </para>
      <para>
       N-distinct counts for combinations of column values. If greater
       than zero, the estimated number of distinct values in the combination.
       If less than zero, the negative of the number of distinct values divided
       by the number of rows.
       (The negated form is used when <command>ANALYZE</command> believes that
       the number of distinct values is likely to increase as the table grows;
       the positive form is used when the column seems to have a fixed number
       of possible values.)  For example, -1 indicates a unique combination of
       columns in which the number of distinct combinations is the same as the
       number of rows.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>dependencies</structfield> <type>pg_dependencies</type>
      </para>
      <para>
       Functional dependency statistics
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>most_common_vals</structfield> <type>text[]</type>
      </para>
      <para>
       A list of the most common combinations of values in the columns.
       (Null if no combinations seem to be more common than any others.)
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>most_common_val_nulls</structfield> <type>bool[]</type>
      </para>
      <para>
       A list of NULL flags for the most common combinations of values.
       (Null when <structfield>most_common_vals</structfield> is.)
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>most_common_freqs</structfield> <type>float8[]</type>
      </para>
      <para>
       A list of the frequencies of the most common combinations,
       i.e., number of occurrences of each divided by total number of rows.
       (Null when <structfield>most_common_vals</structfield> is.)
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>most_common_base_freqs</structfield> <type>float8[]</type>
      </para>
      <para>
       A list of the base frequencies of the most common combinations,
       i.e., product of per-value frequencies.
       (Null when <structfield>most_common_vals</structfield> is.)
      </para></entry>
     </row>
    </tbody>
   </tgroup>
  </table>

  <para>
   The maximum number of entries in the array fields can be controlled on a
   column-by-column basis using the <link linkend="sql-altertable"><command>ALTER
   TABLE SET STATISTICS</command></link> command, or globally by setting the
   <xref linkend="guc-default-statistics-target"/> run-time parameter.
  </para>

 </sect1>

 <sect1 id="view-pg-stats-ext-exprs">
  <title><structname>pg_stats_ext_exprs</structname></title>

  <indexterm zone="view-pg-stats-ext-exprs">
   <primary>pg_stats_ext_exprs</primary>
  </indexterm>

  <para>
   The view <structname>pg_stats_ext_exprs</structname> provides access to
   information about all expressions included in extended statistics objects,
   combining information stored in the <link
   linkend="catalog-pg-statistic-ext"><structname>pg_statistic_ext</structname></link>
   and <link linkend="catalog-pg-statistic-ext-data"><structname>pg_statistic_ext_data</structname></link>
   catalogs.  This view allows access only to rows of
   <link linkend="catalog-pg-statistic-ext"><structname>pg_statistic_ext</structname></link> and <link linkend="catalog-pg-statistic-ext-data"><structname>pg_statistic_ext_data</structname></link>
   that correspond to tables the user owns, and therefore
   it is safe to allow public read access to this view.
  </para>

  <para>
   <structname>pg_stats_ext_exprs</structname> is also designed to present
   the information in a more readable format than the underlying catalogs
   &mdash; at the cost that its schema must be extended whenever the structure
   of statistics in <structname>pg_statistic_ext</structname> changes.
  </para>

  <table>
   <title><structname>pg_stats_ext_exprs</structname> Columns</title>
   <tgroup cols="1">
    <thead>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       Column Type
      </para>
      <para>
       Description
      </para></entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>schemaname</structfield> <type>name</type>
       (references <link linkend="catalog-pg-namespace"><structname>pg_namespace</structname></link>.<structfield>nspname</structfield>)
      </para>
      <para>
       Name of schema containing table
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>tablename</structfield> <type>name</type>
       (references <link linkend="catalog-pg-class"><structname>pg_class</structname></link>.<structfield>relname</structfield>)
      </para>
      <para>
       Name of table the statistics object is defined on
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>statistics_schemaname</structfield> <type>name</type>
       (references <link linkend="catalog-pg-namespace"><structname>pg_namespace</structname></link>.<structfield>nspname</structfield>)
      </para>
      <para>
       Name of schema containing extended statistics object
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>statistics_name</structfield> <type>name</type>
       (references <link linkend="catalog-pg-statistic-ext"><structname>pg_statistic_ext</structname></link>.<structfield>stxname</structfield>)
      </para>
      <para>
       Name of extended statistics object
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>statistics_owner</structfield> <type>name</type>
       (references <link linkend="catalog-pg-authid"><structname>pg_authid</structname></link>.<structfield>rolname</structfield>)
      </para>
      <para>
       Owner of the extended statistics object
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>expr</structfield> <type>text</type>
      </para>
      <para>
       Expression included in the extended statistics object
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>inherited</structfield> <type>bool</type>
       (references <link linkend="catalog-pg-statistic-ext-data"><structname>pg_statistic_ext_data</structname></link>.<structfield>stxdinherit</structfield>)
      </para>
      <para>
       If true, the stats include values from child tables, not just the
       values in the specified relation
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>null_frac</structfield> <type>float4</type>
      </para>
      <para>
       Fraction of expression entries that are null
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>avg_width</structfield> <type>int4</type>
      </para>
      <para>
       Average width in bytes of expression's entries
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>n_distinct</structfield> <type>float4</type>
      </para>
      <para>
       If greater than zero, the estimated number of distinct values in the
       expression.  If less than zero, the negative of the number of distinct
       values divided by the number of rows.  (The negated form is used when
       <command>ANALYZE</command> believes that the number of distinct values is
       likely to increase as the table grows; the positive form is used when
       the expression seems to have a fixed number of possible values.)  For
       example, -1 indicates a unique expression in which the number of distinct
       values is the same as the number of rows.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>most_common_vals</structfield> <type>anyarray</type>
      </para>
      <para>
       A list of the most common values in the expression. (Null if
       no values seem to be more common than any others.)
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>most_common_freqs</structfield> <type>float4[]</type>
      </para>
      <para>
       A list of the frequencies of the most common values,
       i.e., number of occurrences of each divided by total number of rows.
       (Null when <structfield>most_common_vals</structfield> is.)
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>histogram_bounds</structfield> <type>anyarray</type>
      </para>
      <para>
       A list of values that divide the expression's values into groups of
       approximately equal population.  The values in
       <structfield>most_common_vals</structfield>, if present, are omitted from this
       histogram calculation.  (This expression is null if the expression data type
       does not have a <literal>&lt;</literal> operator or if the
       <structfield>most_common_vals</structfield> list accounts for the entire
       population.)
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>correlation</structfield> <type>float4</type>
      </para>
      <para>
       Statistical correlation between physical row ordering and
       logical ordering of the expression values.  This ranges from -1 to +1.
       When the value is near -1 or +1, an index scan on the expression will
       be estimated to be cheaper than when it is near zero, due to reduction
       of random access to the disk.  (This expression is null if the expression's
       data type does not have a <literal>&lt;</literal> operator.)
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>most_common_elems</structfield> <type>anyarray</type>
      </para>
      <para>
       A list of non-null element values most often appearing within values of
       the expression. (Null for scalar types.)
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>most_common_elem_freqs</structfield> <type>float4[]</type>
      </para>
      <para>
       A list of the frequencies of the most common element values, i.e., the
       fraction of rows containing at least one instance of the given value.
       Two or three additional values follow the per-element frequencies;
       these are the minimum and maximum of the preceding per-element
       frequencies, and optionally the frequency of null elements.
       (Null when <structfield>most_common_elems</structfield> is.)
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>elem_count_histogram</structfield> <type>float4[]</type>
      </para>
      <para>
       A histogram of the counts of distinct non-null element values within the
       values of the expression, followed by the average number of distinct
       non-null elements.  (Null for scalar types.)
      </para></entry>
     </row>
    </tbody>
   </tgroup>
  </table>

  <para>
   The maximum number of entries in the array fields can be controlled on a
   column-by-column basis using the <link linkend="sql-altertable"><command>ALTER
   TABLE SET STATISTICS</command></link> command, or globally by setting the
   <xref linkend="guc-default-statistics-target"/> run-time parameter.
  </para>

 </sect1>

 <sect1 id="view-pg-tables">
  <title><structname>pg_tables</structname></title>

  <indexterm zone="view-pg-tables">
   <primary>pg_tables</primary>
  </indexterm>

  <para>
   The view <structname>pg_tables</structname> provides access to
   useful information about each table in the database.
  </para>

  <table>
   <title><structname>pg_tables</structname> Columns</title>
   <tgroup cols="1">
    <thead>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       Column Type
      </para>
      <para>
       Description
      </para></entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>schemaname</structfield> <type>name</type>
       (references <link linkend="catalog-pg-namespace"><structname>pg_namespace</structname></link>.<structfield>nspname</structfield>)
      </para>
      <para>
       Name of schema containing table
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>tablename</structfield> <type>name</type>
       (references <link linkend="catalog-pg-class"><structname>pg_class</structname></link>.<structfield>relname</structfield>)
      </para>
      <para>
       Name of table
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>tableowner</structfield> <type>name</type>
       (references <link linkend="catalog-pg-authid"><structname>pg_authid</structname></link>.<structfield>rolname</structfield>)
      </para>
      <para>
       Name of table's owner
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>tablespace</structfield> <type>name</type>
       (references <link linkend="catalog-pg-tablespace"><structname>pg_tablespace</structname></link>.<structfield>spcname</structfield>)
      </para>
      <para>
       Name of tablespace containing table (null if default for database)
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>hasindexes</structfield> <type>bool</type>
       (references <link linkend="catalog-pg-class"><structname>pg_class</structname></link>.<structfield>relhasindex</structfield>)
      </para>
      <para>
       True if table has (or recently had) any indexes
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>hasrules</structfield> <type>bool</type>
       (references <link linkend="catalog-pg-class"><structname>pg_class</structname></link>.<structfield>relhasrules</structfield>)
      </para>
      <para>
       True if table has (or once had) rules
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>hastriggers</structfield> <type>bool</type>
       (references <link linkend="catalog-pg-class"><structname>pg_class</structname></link>.<structfield>relhastriggers</structfield>)
      </para>
      <para>
       True if table has (or once had) triggers
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>rowsecurity</structfield> <type>bool</type>
       (references <link linkend="catalog-pg-class"><structname>pg_class</structname></link>.<structfield>relrowsecurity</structfield>)
      </para>
      <para>
       True if row security is enabled on the table
      </para></entry>
     </row>
    </tbody>
   </tgroup>
  </table>

 </sect1>

 <sect1 id="view-pg-timezone-abbrevs">
  <title><structname>pg_timezone_abbrevs</structname></title>

  <indexterm zone="view-pg-timezone-abbrevs">
   <primary>pg_timezone_abbrevs</primary>
  </indexterm>

  <para>
   The view <structname>pg_timezone_abbrevs</structname> provides a list
   of time zone abbreviations that are currently recognized by the datetime
   input routines.  The contents of this view change when the
   <xref linkend="guc-timezone-abbreviations"/> run-time parameter is modified.
  </para>

  <table>
   <title><structname>pg_timezone_abbrevs</structname> Columns</title>
   <tgroup cols="1">
    <thead>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       Column Type
      </para>
      <para>
       Description
      </para></entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>abbrev</structfield> <type>text</type>
      </para>
      <para>
       Time zone abbreviation
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>utc_offset</structfield> <type>interval</type>
      </para>
      <para>
       Offset from UTC (positive means east of Greenwich)
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>is_dst</structfield> <type>bool</type>
      </para>
      <para>
       True if this is a daylight-savings abbreviation
      </para></entry>
     </row>
    </tbody>
   </tgroup>
  </table>

  <para>
   While most timezone abbreviations represent fixed offsets from UTC,
   there are some that have historically varied in value
   (see <xref linkend="datetime-config-files"/> for more information).
   In such cases this view presents their current meaning.
  </para>

 </sect1>

 <sect1 id="view-pg-timezone-names">
  <title><structname>pg_timezone_names</structname></title>

  <indexterm zone="view-pg-timezone-names">
   <primary>pg_timezone_names</primary>
  </indexterm>

  <para>
   The view <structname>pg_timezone_names</structname> provides a list
   of time zone names that are recognized by <command>SET TIMEZONE</command>,
   along with their associated abbreviations, UTC offsets,
   and daylight-savings status.  (Technically,
   <productname>PostgreSQL</productname> does not use UTC because leap
   seconds are not handled.)
   Unlike the abbreviations shown in <link
   linkend="view-pg-timezone-abbrevs"><structname>pg_timezone_abbrevs</structname></link>, many of these names imply a set of daylight-savings transition
   date rules.  Therefore, the associated information changes across local DST
   boundaries.  The displayed information is computed based on the current
   value of <function>CURRENT_TIMESTAMP</function>.
  </para>

  <table>
   <title><structname>pg_timezone_names</structname> Columns</title>
   <tgroup cols="1">
    <thead>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       Column Type
      </para>
      <para>
       Description
      </para></entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>name</structfield> <type>text</type>
      </para>
      <para>
       Time zone name
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>abbrev</structfield> <type>text</type>
      </para>
      <para>
       Time zone abbreviation
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>utc_offset</structfield> <type>interval</type>
      </para>
      <para>
       Offset from UTC (positive means east of Greenwich)
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>is_dst</structfield> <type>bool</type>
      </para>
      <para>
       True if currently observing daylight savings
      </para></entry>
     </row>
    </tbody>
   </tgroup>
  </table>

 </sect1>

 <sect1 id="view-pg-user">
  <title><structname>pg_user</structname></title>

  <indexterm zone="view-pg-user">
   <primary>pg_user</primary>
  </indexterm>

  <para>
   The view <structname>pg_user</structname> provides access to
   information about database users.  This is simply a publicly
   readable view of
   <link linkend="view-pg-shadow"><structname>pg_shadow</structname></link>
   that blanks out the password field.
  </para>

  <table>
   <title><structname>pg_user</structname> Columns</title>
   <tgroup cols="1">
    <thead>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       Column Type
      </para>
      <para>
       Description
      </para></entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>usename</structfield> <type>name</type>
      </para>
      <para>
       User name
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>usesysid</structfield> <type>oid</type>
      </para>
      <para>
       ID of this user
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>usecreatedb</structfield> <type>bool</type>
      </para>
      <para>
       User can create databases
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>usesuper</structfield> <type>bool</type>
      </para>
      <para>
       User is a superuser
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>userepl</structfield> <type>bool</type>
      </para>
      <para>
       User can initiate streaming replication and put the system in and
       out of backup mode.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>usebypassrls</structfield> <type>bool</type>
      </para>
      <para>
       User bypasses every row-level security policy, see
       <xref linkend="ddl-rowsecurity"/> for more information.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>passwd</structfield> <type>text</type>
      </para>
      <para>
       Not the password (always reads as <literal>********</literal>)
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>valuntil</structfield> <type>timestamptz</type>
      </para>
      <para>
       Password expiry time (only used for password authentication)
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>useconfig</structfield> <type>text[]</type>
      </para>
      <para>
       Session defaults for run-time configuration variables
      </para></entry>
     </row>
    </tbody>
   </tgroup>
  </table>

 </sect1>

 <sect1 id="view-pg-user-mappings">
  <title><structname>pg_user_mappings</structname></title>

  <indexterm zone="view-pg-user-mappings">
   <primary>pg_user_mappings</primary>
  </indexterm>

  <para>
   The view <structname>pg_user_mappings</structname> provides access
   to information about user mappings.  This is essentially a publicly
   readable view of
   <link linkend="catalog-pg-user-mapping"><structname>pg_user_mapping</structname></link>
   that leaves out the options field if the user has no rights to use
   it.
  </para>

  <table>
   <title><structname>pg_user_mappings</structname> Columns</title>
   <tgroup cols="1">
    <thead>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       Column Type
      </para>
      <para>
       Description
      </para></entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>umid</structfield> <type>oid</type>
       (references <link linkend="catalog-pg-user-mapping"><structname>pg_user_mapping</structname></link>.<structfield>oid</structfield>)
      </para>
      <para>
       OID of the user mapping
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>srvid</structfield> <type>oid</type>
       (references <link linkend="catalog-pg-foreign-server"><structname>pg_foreign_server</structname></link>.<structfield>oid</structfield>)
      </para>
      <para>
       The OID of the foreign server that contains this mapping
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>srvname</structfield> <type>name</type>
       (references <link linkend="catalog-pg-foreign-server"><structname>pg_foreign_server</structname></link>.<structfield>srvname</structfield>)
      </para>
      <para>
       Name of the foreign server
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>umuser</structfield> <type>oid</type>
       (references <link linkend="catalog-pg-authid"><structname>pg_authid</structname></link>.<structfield>oid</structfield>)
      </para>
      <para>
       OID of the local role being mapped, or zero if the user mapping is public
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>usename</structfield> <type>name</type>
      </para>
      <para>
       Name of the local user to be mapped
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>umoptions</structfield> <type>text[]</type>
      </para>
      <para>
       User mapping specific options, as <quote>keyword=value</quote> strings
      </para></entry>
     </row>
    </tbody>
   </tgroup>
  </table>

  <para>
   To protect password information stored as a user mapping option,
   the <structfield>umoptions</structfield> column will read as null
   unless one of the following applies:
   <itemizedlist>
    <listitem>
     <para>
      current user is the user being mapped, and owns the server or
      holds <literal>USAGE</literal> privilege on it
     </para>
    </listitem>
    <listitem>
     <para>
      current user is the server owner and mapping is for <literal>PUBLIC</literal>
     </para>
    </listitem>
    <listitem>
     <para>
      current user is a superuser
     </para>
    </listitem>
   </itemizedlist>
  </para>

 </sect1>


 <sect1 id="view-pg-views">
  <title><structname>pg_views</structname></title>

  <indexterm zone="view-pg-views">
   <primary>pg_views</primary>
  </indexterm>

  <para>
   The view <structname>pg_views</structname> provides access to
   useful information about each view in the database.
  </para>

  <table>
   <title><structname>pg_views</structname> Columns</title>
   <tgroup cols="1">
    <thead>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       Column Type
      </para>
      <para>
       Description
      </para></entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>schemaname</structfield> <type>name</type>
       (references <link linkend="catalog-pg-namespace"><structname>pg_namespace</structname></link>.<structfield>nspname</structfield>)
      </para>
      <para>
       Name of schema containing view
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>viewname</structfield> <type>name</type>
       (references <link linkend="catalog-pg-class"><structname>pg_class</structname></link>.<structfield>relname</structfield>)
      </para>
      <para>
       Name of view
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>viewowner</structfield> <type>name</type>
       (references <link linkend="catalog-pg-authid"><structname>pg_authid</structname></link>.<structfield>rolname</structfield>)
      </para>
      <para>
       Name of view's owner
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>definition</structfield> <type>text</type>
      </para>
      <para>
       View definition (a reconstructed <xref linkend="sql-select"/> query)
      </para></entry>
     </row>
    </tbody>
   </tgroup>
  </table>
 </sect1>


 <sect1 id="view-pg-wait-events">
  <title><structname>pg_wait_events</structname></title>

  <indexterm zone="view-pg-wait-events">
   <primary>pg_wait_events</primary>
  </indexterm>

  <para>
   The view <structname>pg_wait_events</structname> provides description about the
   wait events.
  </para>

  <table>
   <title><structname>pg_wait_events</structname> Columns</title>
   <tgroup cols="1">
    <thead>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       Column Type
      </para>
      <para>
       Description
      </para></entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>type</structfield> <type>text</type>
      </para>
      <para>
       Wait event type
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>name</structfield> <type>text</type>
      </para>
      <para>
       Wait event name
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>description</structfield> <type>text</type>
      </para>
      <para>
       Wait event description
      </para></entry>
     </row>
    </tbody>
   </tgroup>
  </table>
 </sect1>

</chapter>
