Adding upstream version 14.5.upstream/14.5upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 864 insertions, 0 deletions

diff --git a/doc/src/sgml/pgstatstatements.sgml b/doc/src/sgml/pgstatstatements.sgml
new file mode 100644
index 0000000..70b28a9
--- /dev/null
+++ b/doc/src/sgml/pgstatstatements.sgml
@@ -0,0 +1,864 @@
+<!-- doc/src/sgml/pgstatstatements.sgml -->
+
+<sect1 id="pgstatstatements" xreflabel="pg_stat_statements">
+ <title>pg_stat_statements</title>
+
+ <indexterm zone="pgstatstatements">
+  <primary>pg_stat_statements</primary>
+ </indexterm>
+
+ <para>
+  The <filename>pg_stat_statements</filename> module provides a means for
+  tracking planning and execution statistics of all SQL statements executed by
+  a server.
+ </para>
+
+ <para>
+  The module must be loaded by adding <literal>pg_stat_statements</literal> to
+  <xref linkend="guc-shared-preload-libraries"/> in
+  <filename>postgresql.conf</filename>, because it requires additional shared memory.
+  This means that a server restart is needed to add or remove the module.
+  In addition, query identifier calculation must be enabled in order for the
+  module to be active, which is done automatically if <xref linkend="guc-compute-query-id"/>
+  is set to <literal>auto</literal> or <literal>on</literal>, or any third-party
+  module that calculates query identifiers is loaded.
+ </para>
+
+ <para>
+   When <filename>pg_stat_statements</filename> is active, it tracks
+   statistics across all databases of the server.  To access and manipulate
+   these statistics, the module provides views
+   <structname>pg_stat_statements</structname> and
+   <structname>pg_stat_statements_info</structname>,
+   and the utility functions <function>pg_stat_statements_reset</function> and
+   <function>pg_stat_statements</function>.  These are not available globally but
+   can be enabled for a specific database with
+   <command>CREATE EXTENSION pg_stat_statements</command>.
+ </para>
+
+ <sect2>
+  <title>The <structname>pg_stat_statements</structname> View</title>
+
+  <para>
+   The statistics gathered by the module are made available via a
+   view named <structname>pg_stat_statements</structname>.  This view
+   contains one row for each distinct combination of database ID, user
+   ID, query ID and whether it's a top-level statement or not (up to
+   the maximum number of distinct statements that the module can track).
+   The columns of the view are shown in
+   <xref linkend="pgstatstatements-columns"/>.
+  </para>
+
+  <table id="pgstatstatements-columns">
+   <title><structname>pg_stat_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>userid</structfield> <type>oid</type>
+       (references <link linkend="catalog-pg-authid"><structname>pg_authid</structname></link>.<structfield>oid</structfield>)
+      </para>
+      <para>
+       OID of user who executed the statement
+      </para></entry>
+     </row>
+
+     <row>
+      <entry role="catalog_table_entry"><para role="column_definition">
+       <structfield>dbid</structfield> <type>oid</type>
+       (references <link linkend="catalog-pg-database"><structname>pg_database</structname></link>.<structfield>oid</structfield>)
+      </para>
+      <para>
+       OID of database in which the statement was executed
+      </para></entry>
+     </row>
+
+     <row>
+      <entry role="catalog_table_entry"><para role="column_definition">
+       <structfield>toplevel</structfield> <type>bool</type>
+      </para>
+      <para>
+       True if the query was executed as a top-level statement
+       (always true if <varname>pg_stat_statements.track</varname> is set to
+       <literal>top</literal>)
+      </para></entry>
+     </row>
+
+     <row>
+      <entry role="catalog_table_entry"><para role="column_definition">
+       <structfield>queryid</structfield> <type>bigint</type>
+      </para>
+      <para>
+       Hash code to identify identical normalized queries.
+      </para></entry>
+     </row>
+
+     <row>
+      <entry role="catalog_table_entry"><para role="column_definition">
+       <structfield>query</structfield> <type>text</type>
+      </para>
+      <para>
+       Text of a representative statement
+      </para></entry>
+     </row>
+
+     <row>
+      <entry role="catalog_table_entry"><para role="column_definition">
+       <structfield>plans</structfield> <type>bigint</type>
+      </para>
+      <para>
+       Number of times the statement was planned
+       (if <varname>pg_stat_statements.track_planning</varname> is enabled,
+       otherwise zero)
+      </para></entry>
+     </row>
+
+     <row>
+      <entry role="catalog_table_entry"><para role="column_definition">
+       <structfield>total_plan_time</structfield> <type>double precision</type>
+      </para>
+      <para>
+       Total time spent planning the statement, in milliseconds
+       (if <varname>pg_stat_statements.track_planning</varname> is enabled,
+       otherwise zero)
+      </para></entry>
+     </row>
+
+     <row>
+      <entry role="catalog_table_entry"><para role="column_definition">
+       <structfield>min_plan_time</structfield> <type>double precision</type>
+      </para>
+      <para>
+       Minimum time spent planning the statement, in milliseconds
+       (if <varname>pg_stat_statements.track_planning</varname> is enabled,
+       otherwise zero)
+      </para></entry>
+     </row>
+
+     <row>
+      <entry role="catalog_table_entry"><para role="column_definition">
+       <structfield>max_plan_time</structfield> <type>double precision</type>
+      </para>
+      <para>
+       Maximum time spent planning the statement, in milliseconds
+       (if <varname>pg_stat_statements.track_planning</varname> is enabled,
+       otherwise zero)
+      </para></entry>
+     </row>
+
+     <row>
+      <entry role="catalog_table_entry"><para role="column_definition">
+       <structfield>mean_plan_time</structfield> <type>double precision</type>
+      </para>
+      <para>
+       Mean time spent planning the statement, in milliseconds
+       (if <varname>pg_stat_statements.track_planning</varname> is enabled,
+       otherwise zero)
+      </para></entry>
+     </row>
+
+     <row>
+      <entry role="catalog_table_entry"><para role="column_definition">
+       <structfield>stddev_plan_time</structfield> <type>double precision</type>
+      </para>
+      <para>
+       Population standard deviation of time spent planning the statement,
+       in milliseconds
+       (if <varname>pg_stat_statements.track_planning</varname> is enabled,
+       otherwise zero)
+      </para></entry>
+     </row>
+
+     <row>
+      <entry role="catalog_table_entry"><para role="column_definition">
+       <structfield>calls</structfield> <type>bigint</type>
+      </para>
+      <para>
+       Number of times the statement was executed
+      </para></entry>
+     </row>
+
+     <row>
+      <entry role="catalog_table_entry"><para role="column_definition">
+       <structfield>total_exec_time</structfield> <type>double precision</type>
+      </para>
+      <para>
+       Total time spent executing the statement, in milliseconds
+      </para></entry>
+     </row>
+
+     <row>
+      <entry role="catalog_table_entry"><para role="column_definition">
+       <structfield>min_exec_time</structfield> <type>double precision</type>
+      </para>
+      <para>
+       Minimum time spent executing the statement, in milliseconds
+      </para></entry>
+     </row>
+
+     <row>
+      <entry role="catalog_table_entry"><para role="column_definition">
+       <structfield>max_exec_time</structfield> <type>double precision</type>
+      </para>
+      <para>
+       Maximum time spent executing the statement, in milliseconds
+      </para></entry>
+     </row>
+
+     <row>
+      <entry role="catalog_table_entry"><para role="column_definition">
+       <structfield>mean_exec_time</structfield> <type>double precision</type>
+      </para>
+      <para>
+       Mean time spent executing the statement, in milliseconds
+      </para></entry>
+     </row>
+
+     <row>
+      <entry role="catalog_table_entry"><para role="column_definition">
+       <structfield>stddev_exec_time</structfield> <type>double precision</type>
+      </para>
+      <para>
+       Population standard deviation of time spent executing the statement, in milliseconds
+      </para></entry>
+     </row>
+
+     <row>
+      <entry role="catalog_table_entry"><para role="column_definition">
+       <structfield>rows</structfield> <type>bigint</type>
+      </para>
+      <para>
+       Total number of rows retrieved or affected by the statement
+      </para></entry>
+     </row>
+
+     <row>
+      <entry role="catalog_table_entry"><para role="column_definition">
+       <structfield>shared_blks_hit</structfield> <type>bigint</type>
+      </para>
+      <para>
+       Total number of shared block cache hits by the statement
+      </para></entry>
+     </row>
+
+     <row>
+      <entry role="catalog_table_entry"><para role="column_definition">
+       <structfield>shared_blks_read</structfield> <type>bigint</type>
+      </para>
+      <para>
+       Total number of shared blocks read by the statement
+      </para></entry>
+     </row>
+
+     <row>
+      <entry role="catalog_table_entry"><para role="column_definition">
+       <structfield>shared_blks_dirtied</structfield> <type>bigint</type>
+      </para>
+      <para>
+       Total number of shared blocks dirtied by the statement
+      </para></entry>
+     </row>
+
+     <row>
+      <entry role="catalog_table_entry"><para role="column_definition">
+       <structfield>shared_blks_written</structfield> <type>bigint</type>
+      </para>
+      <para>
+       Total number of shared blocks written by the statement
+      </para></entry>
+     </row>
+
+     <row>
+      <entry role="catalog_table_entry"><para role="column_definition">
+       <structfield>local_blks_hit</structfield> <type>bigint</type>
+      </para>
+      <para>
+       Total number of local block cache hits by the statement
+      </para></entry>
+     </row>
+
+     <row>
+      <entry role="catalog_table_entry"><para role="column_definition">
+       <structfield>local_blks_read</structfield> <type>bigint</type>
+      </para>
+      <para>
+       Total number of local blocks read by the statement
+      </para></entry>
+     </row>
+
+     <row>
+      <entry role="catalog_table_entry"><para role="column_definition">
+       <structfield>local_blks_dirtied</structfield> <type>bigint</type>
+      </para>
+      <para>
+       Total number of local blocks dirtied by the statement
+      </para></entry>
+     </row>
+
+     <row>
+      <entry role="catalog_table_entry"><para role="column_definition">
+       <structfield>local_blks_written</structfield> <type>bigint</type>
+      </para>
+      <para>
+       Total number of local blocks written by the statement
+      </para></entry>
+     </row>
+
+     <row>
+      <entry role="catalog_table_entry"><para role="column_definition">
+       <structfield>temp_blks_read</structfield> <type>bigint</type>
+      </para>
+      <para>
+       Total number of temp blocks read by the statement
+      </para></entry>
+     </row>
+
+     <row>
+      <entry role="catalog_table_entry"><para role="column_definition">
+       <structfield>temp_blks_written</structfield> <type>bigint</type>
+      </para>
+      <para>
+       Total number of temp blocks written by the statement
+      </para></entry>
+     </row>
+
+     <row>
+      <entry role="catalog_table_entry"><para role="column_definition">
+       <structfield>blk_read_time</structfield> <type>double precision</type>
+      </para>
+      <para>
+       Total time the statement spent reading blocks, in milliseconds
+       (if <xref linkend="guc-track-io-timing"/> is enabled, otherwise zero)
+      </para></entry>
+     </row>
+
+     <row>
+      <entry role="catalog_table_entry"><para role="column_definition">
+       <structfield>blk_write_time</structfield> <type>double precision</type>
+      </para>
+      <para>
+       Total time the statement spent writing blocks, in milliseconds
+       (if <xref linkend="guc-track-io-timing"/> is enabled, otherwise zero)
+      </para></entry>
+     </row>
+
+     <row>
+      <entry role="catalog_table_entry"><para role="column_definition">
+       <structfield>wal_records</structfield> <type>bigint</type>
+      </para>
+      <para>
+       Total number of WAL records generated by the statement
+      </para></entry>
+     </row>
+
+     <row>
+      <entry role="catalog_table_entry"><para role="column_definition">
+       <structfield>wal_fpi</structfield> <type>bigint</type>
+      </para>
+      <para>
+       Total number of WAL full page images generated by the statement
+      </para></entry>
+     </row>
+
+     <row>
+      <entry role="catalog_table_entry"><para role="column_definition">
+       <structfield>wal_bytes</structfield> <type>numeric</type>
+      </para>
+      <para>
+       Total amount of WAL generated by the statement in bytes
+      </para></entry>
+     </row>
+    </tbody>
+   </tgroup>
+  </table>
+
+  <para>
+   For security reasons, only superusers and members of the
+   <literal>pg_read_all_stats</literal> role are allowed to see the SQL text and
+   <structfield>queryid</structfield> of queries executed by other users.
+   Other users can see the statistics, however, if the view has been installed
+   in their database.
+  </para>
+
+  <para>
+   Plannable queries (that is, <command>SELECT</command>, <command>INSERT</command>,
+   <command>UPDATE</command>, and <command>DELETE</command>) are combined into a single
+   <structname>pg_stat_statements</structname> entry whenever they have identical query
+   structures according to an internal hash calculation.  Typically, two
+   queries will be considered the same for this purpose if they are
+   semantically equivalent except for the values of literal constants
+   appearing in the query.  Utility commands (that is, all other commands)
+   are compared strictly on the basis of their textual query strings, however.
+  </para>
+
+  <note>
+   <para>
+    The following details about constant replacement and
+    <structfield>queryid</structfield> only apply when <xref
+    linkend="guc-compute-query-id"/> is enabled.  If you use an external
+    module instead to compute <structfield>queryid</structfield>, you
+    should refer to its documentation for details.
+   </para>
+  </note>
+
+  <para>
+   When a constant's value has been ignored for purposes of matching the query
+   to other queries, the constant is replaced by a parameter symbol, such
+   as <literal>$1</literal>, in the <structname>pg_stat_statements</structname>
+   display.
+   The rest of the query text is that of the first query that had the
+   particular <structfield>queryid</structfield> hash value associated with the
+   <structname>pg_stat_statements</structname> entry.
+  </para>
+
+  <para>
+   In some cases, queries with visibly different texts might get merged into a
+   single <structname>pg_stat_statements</structname> entry.  Normally this will happen
+   only for semantically equivalent queries, but there is a small chance of
+   hash collisions causing unrelated queries to be merged into one entry.
+   (This cannot happen for queries belonging to different users or databases,
+   however.)
+  </para>
+
+  <para>
+   Since the <structfield>queryid</structfield> hash value is computed on the
+   post-parse-analysis representation of the queries, the opposite is
+   also possible: queries with identical texts might appear as
+   separate entries, if they have different meanings as a result of
+   factors such as different <varname>search_path</varname> settings.
+  </para>
+
+  <para>
+   Consumers of <structname>pg_stat_statements</structname> may wish to use
+   <structfield>queryid</structfield> (perhaps in combination with
+   <structfield>dbid</structfield> and <structfield>userid</structfield>) as a more stable
+   and reliable identifier for each entry than its query text.
+   However, it is important to understand that there are only limited
+   guarantees around the stability of the <structfield>queryid</structfield> hash
+   value.  Since the identifier is derived from the
+   post-parse-analysis tree, its value is a function of, among other
+   things, the internal object identifiers appearing in this representation.
+   This has some counterintuitive implications.  For example,
+   <filename>pg_stat_statements</filename> will consider two apparently-identical
+   queries to be distinct, if they reference a table that was dropped
+   and recreated between the executions of the two queries.
+   The hashing process is also sensitive to differences in
+   machine architecture and other facets of the platform.
+   Furthermore, it is not safe to assume that <structfield>queryid</structfield>
+   will be stable across major versions of <productname>PostgreSQL</productname>.
+  </para>
+
+  <para>
+   As a rule of thumb, <structfield>queryid</structfield> values can be assumed to be
+   stable and comparable only so long as the underlying server version and
+   catalog metadata details stay exactly the same.  Two servers
+   participating in replication based on physical WAL replay can be expected
+   to have identical <structfield>queryid</structfield> values for the same query.
+   However, logical replication schemes do not promise to keep replicas
+   identical in all relevant details, so <structfield>queryid</structfield> will
+   not be a useful identifier for accumulating costs across a set of logical
+   replicas.  If in doubt, direct testing is recommended.
+  </para>
+
+  <para>
+   The parameter symbols used to replace constants in
+   representative query texts start from the next number after the
+   highest <literal>$</literal><replaceable>n</replaceable> parameter in the original query
+   text, or <literal>$1</literal> if there was none.  It's worth noting that in
+   some cases there may be hidden parameter symbols that affect this
+   numbering.  For example, <application>PL/pgSQL</application> uses hidden parameter
+   symbols to insert values of function local variables into queries, so that
+   a <application>PL/pgSQL</application> statement like <literal>SELECT i + 1 INTO j</literal>
+   would have representative text like <literal>SELECT i + $2</literal>.
+  </para>
+
+  <para>
+   The representative query texts are kept in an external disk file, and do
+   not consume shared memory.  Therefore, even very lengthy query texts can
+   be stored successfully.  However, if many long query texts are
+   accumulated, the external file might grow unmanageably large.  As a
+   recovery method if that happens, <filename>pg_stat_statements</filename> may
+   choose to discard the query texts, whereupon all existing entries in
+   the <structname>pg_stat_statements</structname> view will show
+   null <structfield>query</structfield> fields, though the statistics associated with
+   each <structfield>queryid</structfield> are preserved.  If this happens, consider
+   reducing <varname>pg_stat_statements.max</varname> to prevent
+   recurrences.
+  </para>
+
+  <para>
+   <structfield>plans</structfield> and <structfield>calls</structfield> aren't
+   always expected to match because planning and execution statistics are
+   updated at their respective end phase, and only for successful operations.
+   For example, if a statement is successfully planned but fails during
+   the execution phase, only its planning statistics will be updated.
+   If planning is skipped because a cached plan is used, only its execution
+   statistics will be updated.
+  </para>
+ </sect2>
+
+ <sect2>
+  <title>The <structname>pg_stat_statements_info</structname> View</title>
+
+  <indexterm>
+   <primary>pg_stat_statements_info</primary>
+  </indexterm>
+
+  <para>
+   The statistics of the <filename>pg_stat_statements</filename> module
+   itself are tracked and made available via a view named
+   <structname>pg_stat_statements_info</structname>.  This view contains
+   only a single row.  The columns of the view are shown in
+   <xref linkend="pgstatstatementsinfo-columns"/>.
+  </para>
+
+  <table id="pgstatstatementsinfo-columns">
+   <title><structname>pg_stat_statements_info</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>dealloc</structfield> <type>bigint</type>
+      </para>
+      <para>
+       Total number of times <structname>pg_stat_statements</structname>
+       entries about the least-executed statements were deallocated
+       because more distinct statements than
+       <varname>pg_stat_statements.max</varname> were observed
+      </para></entry>
+     </row>
+     <row>
+      <entry role="catalog_table_entry"><para role="column_definition">
+       <structfield>stats_reset</structfield> <type>timestamp with time zone</type>
+      </para>
+      <para>
+       Time at which all statistics in the
+       <structname>pg_stat_statements</structname> view were last reset.
+      </para></entry>
+     </row>
+
+    </tbody>
+   </tgroup>
+  </table>
+ </sect2>
+
+ <sect2>
+  <title>Functions</title>
+
+  <variablelist>
+   <varlistentry>
+    <term>
+     <function>pg_stat_statements_reset(userid Oid, dbid Oid, queryid bigint) returns void</function>
+     <indexterm>
+      <primary>pg_stat_statements_reset</primary>
+     </indexterm>
+    </term>
+
+    <listitem>
+     <para>
+      <function>pg_stat_statements_reset</function> discards statistics
+      gathered so far by <filename>pg_stat_statements</filename> corresponding
+      to the specified <structfield>userid</structfield>, <structfield>dbid</structfield>
+      and <structfield>queryid</structfield>.  If any of the parameters are not
+      specified, the default value <literal>0</literal>(invalid) is used for
+      each of them and the statistics that match with other parameters will be
+      reset.  If no parameter is specified or all the specified parameters are
+      <literal>0</literal>(invalid), it will discard all statistics.
+      If all statistics in the <filename>pg_stat_statements</filename>
+      view are discarded, it will also reset the statistics in the
+      <structname>pg_stat_statements_info</structname> view.
+      By default, this function can only be executed by superusers.
+      Access may be granted to others using <command>GRANT</command>.
+     </para>
+    </listitem>
+   </varlistentry>
+
+   <varlistentry>
+    <term>
+     <function>pg_stat_statements(showtext boolean) returns setof record</function>
+     <indexterm>
+      <primary>pg_stat_statements</primary>
+      <secondary>function</secondary>
+     </indexterm>
+    </term>
+
+    <listitem>
+     <para>
+      The <structname>pg_stat_statements</structname> view is defined in
+      terms of a function also named <function>pg_stat_statements</function>.
+      It is possible for clients to call
+      the <function>pg_stat_statements</function> function directly, and by
+      specifying <literal>showtext := false</literal> have query text be
+      omitted (that is, the <literal>OUT</literal> argument that corresponds
+      to the view's <structfield>query</structfield> column will return nulls).  This
+      feature is intended to support external tools that might wish to avoid
+      the overhead of repeatedly retrieving query texts of indeterminate
+      length.  Such tools can instead cache the first query text observed
+      for each entry themselves, since that is
+      all <filename>pg_stat_statements</filename> itself does, and then retrieve
+      query texts only as needed.  Since the server stores query texts in a
+      file, this approach may reduce physical I/O for repeated examination
+      of the <structname>pg_stat_statements</structname> data.
+     </para>
+    </listitem>
+   </varlistentry>
+  </variablelist>
+ </sect2>
+
+ <sect2>
+  <title>Configuration Parameters</title>
+
+  <variablelist>
+   <varlistentry>
+    <term>
+     <varname>pg_stat_statements.max</varname> (<type>integer</type>)
+    </term>
+
+    <listitem>
+     <para>
+      <varname>pg_stat_statements.max</varname> is the maximum number of
+      statements tracked by the module (i.e., the maximum number of rows
+      in the <structname>pg_stat_statements</structname> view).  If more distinct
+      statements than that are observed, information about the least-executed
+      statements is discarded.  The number of times such information was
+      discarded can be seen in the
+      <structname>pg_stat_statements_info</structname> view.
+      The default value is 5000.
+      This parameter can only be set at server start.
+     </para>
+    </listitem>
+   </varlistentry>
+
+   <varlistentry>
+    <term>
+     <varname>pg_stat_statements.track</varname> (<type>enum</type>)
+    </term>
+
+    <listitem>
+     <para>
+      <varname>pg_stat_statements.track</varname> controls which statements
+      are counted by the module.
+      Specify <literal>top</literal> to track top-level statements (those issued
+      directly by clients), <literal>all</literal> to also track nested statements
+      (such as statements invoked within functions), or <literal>none</literal> to
+      disable statement statistics collection.
+      The default value is <literal>top</literal>.
+      Only superusers can change this setting.
+     </para>
+    </listitem>
+   </varlistentry>
+
+   <varlistentry>
+    <term>
+     <varname>pg_stat_statements.track_utility</varname> (<type>boolean</type>)
+    </term>
+
+    <listitem>
+     <para>
+      <varname>pg_stat_statements.track_utility</varname> controls whether
+      utility commands are tracked by the module.  Utility commands are
+      all those other than <command>SELECT</command>, <command>INSERT</command>,
+      <command>UPDATE</command> and <command>DELETE</command>.
+      The default value is <literal>on</literal>.
+      Only superusers can change this setting.
+     </para>
+    </listitem>
+   </varlistentry>
+
+   <varlistentry>
+    <term>
+     <varname>pg_stat_statements.track_planning</varname> (<type>boolean</type>)
+    </term>
+
+    <listitem>
+     <para>
+      <varname>pg_stat_statements.track_planning</varname> controls whether
+      planning operations and duration are tracked by the module.
+      Enabling this parameter may incur a noticeable performance penalty,
+      especially when statements with identical query structure are executed
+      by many concurrent connections which compete to update a small number of
+      <structname>pg_stat_statements</structname> entries.
+      The default value is <literal>off</literal>.
+      Only superusers can change this setting.
+     </para>
+    </listitem>
+   </varlistentry>
+
+   <varlistentry>
+    <term>
+     <varname>pg_stat_statements.save</varname> (<type>boolean</type>)
+    </term>
+
+    <listitem>
+     <para>
+      <varname>pg_stat_statements.save</varname> specifies whether to
+      save statement statistics across server shutdowns.
+      If it is <literal>off</literal> then statistics are not saved at
+      shutdown nor reloaded at server start.
+      The default value is <literal>on</literal>.
+      This parameter can only be set in the <filename>postgresql.conf</filename>
+      file or on the server command line.
+     </para>
+    </listitem>
+   </varlistentry>
+  </variablelist>
+
+  <para>
+   The module requires additional shared memory proportional to
+   <varname>pg_stat_statements.max</varname>.  Note that this
+   memory is consumed whenever the module is loaded, even if
+   <varname>pg_stat_statements.track</varname> is set to <literal>none</literal>.
+  </para>
+
+  <para>
+   These parameters must be set in <filename>postgresql.conf</filename>.
+   Typical usage might be:
+
+<programlisting>
+# postgresql.conf
+shared_preload_libraries = 'pg_stat_statements'
+
+compute_query_id = on
+pg_stat_statements.max = 10000
+pg_stat_statements.track = all
+</programlisting>
+  </para>
+ </sect2>
+
+ <sect2>
+  <title>Sample Output</title>
+
+<screen>
+bench=# SELECT pg_stat_statements_reset();
+
+$ pgbench -i bench
+$ pgbench -c10 -t300 bench
+
+bench=# \x
+bench=# SELECT query, calls, total_exec_time, rows, 100.0 * shared_blks_hit /
+               nullif(shared_blks_hit + shared_blks_read, 0) AS hit_percent
+          FROM pg_stat_statements ORDER BY total_exec_time DESC LIMIT 5;
+-[ RECORD 1 ]---+--------------------------------------------------&zwsp;------------------
+query           | UPDATE pgbench_branches SET bbalance = bbalance + $1 WHERE bid = $2
+calls           | 3000
+total_exec_time | 25565.855387
+rows            | 3000
+hit_percent     | 100.0000000000000000
+-[ RECORD 2 ]---+--------------------------------------------------&zwsp;------------------
+query           | UPDATE pgbench_tellers SET tbalance = tbalance + $1 WHERE tid = $2
+calls           | 3000
+total_exec_time | 20756.669379
+rows            | 3000
+hit_percent     | 100.0000000000000000
+-[ RECORD 3 ]---+--------------------------------------------------&zwsp;------------------
+query           | copy pgbench_accounts from stdin
+calls           | 1
+total_exec_time | 291.865911
+rows            | 100000
+hit_percent     | 100.0000000000000000
+-[ RECORD 4 ]---+--------------------------------------------------&zwsp;------------------
+query           | UPDATE pgbench_accounts SET abalance = abalance + $1 WHERE aid = $2
+calls           | 3000
+total_exec_time | 271.232977
+rows            | 3000
+hit_percent     | 98.8454011741682975
+-[ RECORD 5 ]---+--------------------------------------------------&zwsp;------------------
+query           | alter table pgbench_accounts add primary key (aid)
+calls           | 1
+total_exec_time | 160.588563
+rows            | 0
+hit_percent     | 100.0000000000000000
+
+
+bench=# SELECT pg_stat_statements_reset(0,0,s.queryid) FROM pg_stat_statements AS s
+            WHERE s.query = 'UPDATE pgbench_branches SET bbalance = bbalance + $1 WHERE bid = $2';
+
+bench=# SELECT query, calls, total_exec_time, rows, 100.0 * shared_blks_hit /
+               nullif(shared_blks_hit + shared_blks_read, 0) AS hit_percent
+          FROM pg_stat_statements ORDER BY total_exec_time DESC LIMIT 5;
+-[ RECORD 1 ]---+--------------------------------------------------&zwsp;------------------
+query           | UPDATE pgbench_tellers SET tbalance = tbalance + $1 WHERE tid = $2
+calls           | 3000
+total_exec_time | 20756.669379
+rows            | 3000
+hit_percent     | 100.0000000000000000
+-[ RECORD 2 ]---+--------------------------------------------------&zwsp;------------------
+query           | copy pgbench_accounts from stdin
+calls           | 1
+total_exec_time | 291.865911
+rows            | 100000
+hit_percent     | 100.0000000000000000
+-[ RECORD 3 ]---+--------------------------------------------------&zwsp;------------------
+query           | UPDATE pgbench_accounts SET abalance = abalance + $1 WHERE aid = $2
+calls           | 3000
+total_exec_time | 271.232977
+rows            | 3000
+hit_percent     | 98.8454011741682975
+-[ RECORD 4 ]---+--------------------------------------------------&zwsp;------------------
+query           | alter table pgbench_accounts add primary key (aid)
+calls           | 1
+total_exec_time | 160.588563
+rows            | 0
+hit_percent     | 100.0000000000000000
+-[ RECORD 5 ]---+--------------------------------------------------&zwsp;------------------
+query           | vacuum analyze pgbench_accounts
+calls           | 1
+total_exec_time | 136.448116
+rows            | 0
+hit_percent     | 99.9201915403032721
+
+bench=# SELECT pg_stat_statements_reset(0,0,0);
+
+bench=# SELECT query, calls, total_exec_time, rows, 100.0 * shared_blks_hit /
+               nullif(shared_blks_hit + shared_blks_read, 0) AS hit_percent
+          FROM pg_stat_statements ORDER BY total_exec_time DESC LIMIT 5;
+-[ RECORD 1 ]---+--------------------------------------------------&zwsp;---------------------------
+query           | SELECT pg_stat_statements_reset(0,0,0)
+calls           | 1
+total_exec_time | 0.189497
+rows            | 1
+hit_percent     | 
+-[ RECORD 2 ]---+--------------------------------------------------&zwsp;---------------------------
+query           | SELECT query, calls, total_exec_time, rows, $1 * shared_blks_hit /          +
+                |                nullif(shared_blks_hit + shared_blks_read, $2) AS hit_percent+
+                |           FROM pg_stat_statements ORDER BY total_exec_time DESC LIMIT $3
+calls           | 0
+total_exec_time | 0
+rows            | 0
+hit_percent     | 
+
+</screen>
+ </sect2>
+
+ <sect2>
+  <title>Authors</title>
+
+  <para>
+   Takahiro Itagaki <email>itagaki.takahiro@oss.ntt.co.jp</email>.
+   Query normalization added by Peter Geoghegan <email>peter@2ndquadrant.com</email>.
+  </para>
+ </sect2>
+
+</sect1>