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7313cc0163
pg_stat_statements now hashes selected fields of the analyzed parse tree to assign a "fingerprint" to each query, and groups all queries with the same fingerprint into a single entry in the pg_stat_statements view. In practice it is expected that queries with the same fingerprint will be equivalent except for values of literal constants. To make the display more useful, such constants are replaced by "?" in the displayed query strings. This mechanism currently supports only optimizable queries (SELECT, INSERT, UPDATE, DELETE). Utility commands are still matched on the basis of their literal query strings. There remain some open questions about how to deal with utility statements that contain optimizable queries (such as EXPLAIN and SELECT INTO) and how to deal with expiring speculative hashtable entries that are made to save the normalized form of a query string. However, fixing these issues should require only localized changes, and since there are other open patches involving contrib/pg_stat_statements, it seems best to go ahead and commit what we've got. Peter Geoghegan, reviewed by Daniel Farina
408 lines
13 KiB
Plaintext
408 lines
13 KiB
Plaintext
<!-- doc/src/sgml/pgstatstatements.sgml -->
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<sect1 id="pgstatstatements" xreflabel="pg_stat_statements">
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<title>pg_stat_statements</title>
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<indexterm zone="pgstatstatements">
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<primary>pg_stat_statements</primary>
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</indexterm>
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<para>
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The <filename>pg_stat_statements</filename> module provides a means for
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tracking execution statistics of all SQL statements executed by a server.
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</para>
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<para>
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The module must be loaded by adding <literal>pg_stat_statements</> to
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<xref linkend="guc-shared-preload-libraries"> in
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<filename>postgresql.conf</>, because it requires additional shared memory.
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This means that a server restart is needed to add or remove the module.
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</para>
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<sect2>
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<title>The <structname>pg_stat_statements</structname> View</title>
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<para>
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The statistics gathered by the module are made available via a system view
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named <structname>pg_stat_statements</>. This view contains one row for
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each distinct query, database ID, and user ID (up to the maximum
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number of distinct statements that the module can track). The columns
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of the view are shown in <xref linkend="pgstatstatements-columns">.
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</para>
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<table id="pgstatstatements-columns">
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<title><structname>pg_stat_statements</> Columns</title>
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<tgroup cols="4">
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<thead>
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<row>
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<entry>Name</entry>
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<entry>Type</entry>
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<entry>References</entry>
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<entry>Description</entry>
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</row>
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</thead>
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<tbody>
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<row>
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<entry><structfield>userid</structfield></entry>
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<entry><type>oid</type></entry>
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<entry><literal><link linkend="catalog-pg-authid"><structname>pg_authid</structname></link>.oid</literal></entry>
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<entry>OID of user who executed the statement</entry>
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</row>
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<row>
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<entry><structfield>dbid</structfield></entry>
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<entry><type>oid</type></entry>
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<entry><literal><link linkend="catalog-pg-database"><structname>pg_database</structname></link>.oid</literal></entry>
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<entry>OID of database in which the statement was executed</entry>
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</row>
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<row>
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<entry><structfield>query</structfield></entry>
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<entry><type>text</type></entry>
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<entry></entry>
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<entry>Text of a representative statement (up to <xref linkend="guc-track-activity-query-size"> bytes)</entry>
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</row>
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<row>
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<entry><structfield>calls</structfield></entry>
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<entry><type>bigint</type></entry>
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<entry></entry>
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<entry>Number of times executed</entry>
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</row>
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<row>
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<entry><structfield>total_time</structfield></entry>
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<entry><type>double precision</type></entry>
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<entry></entry>
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<entry>Total time spent in the statement, in seconds</entry>
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</row>
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<row>
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<entry><structfield>rows</structfield></entry>
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<entry><type>bigint</type></entry>
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<entry></entry>
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<entry>Total number of rows retrieved or affected by the statement</entry>
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</row>
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<row>
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<entry><structfield>shared_blks_hit</structfield></entry>
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<entry><type>bigint</type></entry>
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<entry></entry>
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<entry>Total number of shared blocks hits by the statement</entry>
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</row>
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<row>
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<entry><structfield>shared_blks_read</structfield></entry>
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<entry><type>bigint</type></entry>
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<entry></entry>
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<entry>Total number of shared blocks reads by the statement</entry>
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</row>
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<row>
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<entry><structfield>shared_blks_dirtied</structfield></entry>
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<entry><type>bigint</type></entry>
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<entry></entry>
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<entry>Total number of shared blocks dirtied by the statement</entry>
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</row>
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<row>
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<entry><structfield>shared_blks_written</structfield></entry>
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<entry><type>bigint</type></entry>
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<entry></entry>
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<entry>Total number of shared blocks writes by the statement</entry>
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</row>
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<row>
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<entry><structfield>local_blks_hit</structfield></entry>
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<entry><type>bigint</type></entry>
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<entry></entry>
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<entry>Total number of local blocks hits by the statement</entry>
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</row>
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<row>
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<entry><structfield>local_blks_read</structfield></entry>
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<entry><type>bigint</type></entry>
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<entry></entry>
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<entry>Total number of local blocks reads by the statement</entry>
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</row>
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<row>
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<entry><structfield>local_blks_dirtied</structfield></entry>
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<entry><type>bigint</type></entry>
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<entry></entry>
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<entry>Total number of local blocks dirtied by the statement</entry>
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</row>
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<row>
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<entry><structfield>local_blks_written</structfield></entry>
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<entry><type>bigint</type></entry>
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<entry></entry>
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<entry>Total number of local blocks writes by the statement</entry>
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</row>
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<row>
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<entry><structfield>temp_blks_read</structfield></entry>
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<entry><type>bigint</type></entry>
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<entry></entry>
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<entry>Total number of temp blocks reads by the statement</entry>
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</row>
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<row>
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<entry><structfield>temp_blks_written</structfield></entry>
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<entry><type>bigint</type></entry>
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<entry></entry>
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<entry>Total number of temp blocks writes by the statement</entry>
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</row>
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<row>
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<entry><structfield>time_read</structfield></entry>
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<entry><type>double precision</type></entry>
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<entry></entry>
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<entry>
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Total time the statement spent reading blocks, in seconds
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(if <xref linkend="guc-track-iotiming"> is enabled, otherwise zero)
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</entry>
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</row>
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<row>
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<entry><structfield>time_write</structfield></entry>
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<entry><type>double precision</type></entry>
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<entry></entry>
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<entry>
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Total time the statement spent writing blocks, in seconds
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(if <xref linkend="guc-track-iotiming"> is enabled, otherwise zero)
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</entry>
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</row>
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</tbody>
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</tgroup>
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</table>
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<para>
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This view, and the function <function>pg_stat_statements_reset</>,
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are available only in databases they have been specifically installed into
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by installing the <literal>pg_stat_statements</> extension.
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However, statistics are tracked across all databases of the server
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whenever the <filename>pg_stat_statements</filename> module is loaded
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into the server, regardless of presence of the view.
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</para>
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<para>
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For security reasons, non-superusers are not allowed to see the text of
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queries executed by other users. They can see the statistics, however,
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if the view has been installed in their database.
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</para>
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<para>
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Plannable queries (that is, <command>SELECT</>, <command>INSERT</>,
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<command>UPDATE</>, and <command>DELETE</>) are combined into a single
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<structname>pg_stat_statements</> entry whenever they have identical query
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structures according to an internal hash calculation. Typically, two
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queries will be considered the same for this purpose if they are
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semantically equivalent except for the values of literal constants
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appearing in the query. Utility commands (that is, all other commands)
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are compared strictly on the basis of their textual query strings, however.
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</para>
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<para>
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When a constant's value has been ignored for purposes of matching the
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query to other queries, the constant is replaced by <literal>?</literal>
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in the <structname>pg_stat_statements</> display. The rest of the query
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text is that of the first query that had the particular hash value
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associated with the <structname>pg_stat_statements</> entry.
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</para>
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<para>
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In some cases, queries with visibly different texts might get merged into a
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single <structname>pg_stat_statements</> entry. Normally this will happen
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only for semantically equivalent queries, but there is a small chance of
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hash collisions causing unrelated queries to be merged into one entry.
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(This cannot happen for queries belonging to different users or databases,
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however.)
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</para>
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<para>
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Since the hash value is computed on the post-parse-analysis representation
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of the queries, the opposite is also possible: queries with identical texts
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might appear as separate entries, if they have different meanings as a
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result of factors such as different <varname>search_path</> settings.
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</para>
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</sect2>
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<sect2>
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<title>Functions</title>
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<variablelist>
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<varlistentry>
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<term>
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<function>pg_stat_statements_reset() returns void</function>
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</term>
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<listitem>
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<para>
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<function>pg_stat_statements_reset</function> discards all statistics
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gathered so far by <filename>pg_stat_statements</>.
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By default, this function can only be executed by superusers.
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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</sect2>
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<sect2>
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<title>Configuration Parameters</title>
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<variablelist>
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<varlistentry>
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<term>
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<varname>pg_stat_statements.max</varname> (<type>integer</type>)
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</term>
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<listitem>
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<para>
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<varname>pg_stat_statements.max</varname> is the maximum number of
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statements tracked by the module (i.e., the maximum number of rows
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in the <structname>pg_stat_statements</> view). If more distinct
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statements than that are observed, information about the least-executed
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statements is discarded.
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The default value is 1000.
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This parameter can only be set at server start.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>
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<varname>pg_stat_statements.track</varname> (<type>enum</type>)
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</term>
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<listitem>
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<para>
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<varname>pg_stat_statements.track</varname> controls which statements
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are counted by the module.
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Specify <literal>top</> to track top-level statements (those issued
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directly by clients), <literal>all</> to also track nested statements
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(such as statements invoked within functions), or <literal>none</> to
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disable.
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The default value is <literal>top</>.
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Only superusers can change this setting.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>
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<varname>pg_stat_statements.track_utility</varname> (<type>boolean</type>)
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</term>
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<listitem>
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<para>
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<varname>pg_stat_statements.track_utility</varname> controls whether
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utility commands are tracked by the module. Utility commands are
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all those other than <command>SELECT</>, <command>INSERT</>,
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<command>UPDATE</> and <command>DELETE</>.
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The default value is <literal>on</>.
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Only superusers can change this setting.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>
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<varname>pg_stat_statements.save</varname> (<type>boolean</type>)
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</term>
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<listitem>
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<para>
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<varname>pg_stat_statements.save</varname> specifies whether to
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save statement statistics across server shutdowns.
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If it is <literal>off</> then statistics are not saved at
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shutdown nor reloaded at server start.
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The default value is <literal>on</>.
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This parameter can only be set in the <filename>postgresql.conf</>
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file or on the server command line.
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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<para>
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The module requires additional shared memory amounting to about
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<varname>pg_stat_statements.max</varname> <literal>*</>
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<xref linkend="guc-track-activity-query-size"> bytes. Note that this
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memory is consumed whenever the module is loaded, even if
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<varname>pg_stat_statements.track</> is set to <literal>none</>.
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</para>
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<para>
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These parameters must be set in <filename>postgresql.conf</>.
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Typical usage might be:
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<programlisting>
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# postgresql.conf
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shared_preload_libraries = 'pg_stat_statements'
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pg_stat_statements.max = 10000
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pg_stat_statements.track = all
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</programlisting>
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</para>
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</sect2>
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<sect2>
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<title>Sample Output</title>
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<screen>
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bench=# SELECT pg_stat_statements_reset();
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$ pgbench -i bench
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$ pgbench -c10 -t300 bench
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bench=# \x
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bench=# SELECT query, calls, total_time, rows, 100.0 * shared_blks_hit /
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nullif(shared_blks_hit + shared_blks_read, 0) AS hit_percent
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FROM pg_stat_statements ORDER BY total_time DESC LIMIT 5;
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-[ RECORD 1 ]---------------------------------------------------------------------
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query | UPDATE pgbench_branches SET bbalance = bbalance + ? WHERE bid = ?;
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calls | 3000
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total_time | 9.60900100000002
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rows | 2836
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hit_percent | 99.9778970000200936
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-[ RECORD 2 ]---------------------------------------------------------------------
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query | UPDATE pgbench_tellers SET tbalance = tbalance + ? WHERE tid = ?;
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calls | 3000
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total_time | 8.015156
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rows | 2990
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hit_percent | 99.9731126579631345
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-[ RECORD 3 ]---------------------------------------------------------------------
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query | copy pgbench_accounts from stdin
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calls | 1
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total_time | 0.310624
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rows | 100000
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hit_percent | 0.30395136778115501520
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-[ RECORD 4 ]---------------------------------------------------------------------
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query | UPDATE pgbench_accounts SET abalance = abalance + ? WHERE aid = ?;
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calls | 3000
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total_time | 0.271741999999997
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rows | 3000
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hit_percent | 93.7968855088209426
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-[ RECORD 5 ]---------------------------------------------------------------------
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query | alter table pgbench_accounts add primary key (aid)
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calls | 1
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total_time | 0.08142
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rows | 0
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hit_percent | 34.4947735191637631
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</screen>
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</sect2>
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<sect2>
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<title>Author</title>
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<para>
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Takahiro Itagaki <email>itagaki.takahiro@oss.ntt.co.jp</email>
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</para>
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</sect2>
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</sect1>
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