path: root/doc/src/sgml/pgbuffercache.sgml

<!-- doc/src/sgml/pgbuffercache.sgml -->

<sect1 id="pgbuffercache" xreflabel="pg_buffercache">
 <title>pg_buffercache &mdash; inspect <productname>PostgreSQL</productname>
    buffer cache state</title>

 <indexterm zone="pgbuffercache">
  <primary>pg_buffercache</primary>
 </indexterm>

 <para>
  The <filename>pg_buffercache</filename> module provides a means for
  examining what's happening in the shared buffer cache in real time.
 </para>

 <indexterm>
  <primary>pg_buffercache_pages</primary>
 </indexterm>

 <indexterm>
  <primary>pg_buffercache_summary</primary>
 </indexterm>

 <para>
  This module provides the <function>pg_buffercache_pages()</function>
  function (wrapped in the <structname>pg_buffercache</structname> view),
  the <function>pg_buffercache_summary()</function> function, and the
  <function>pg_buffercache_usage_counts()</function> function.
 </para>

 <para>
  The <function>pg_buffercache_pages()</function> function returns a set of
  records, each row describing the state of one shared buffer entry. The
  <structname>pg_buffercache</structname> view wraps the function for
  convenient use.
 </para>

 <para>
  The <function>pg_buffercache_summary()</function> function returns a single
  row summarizing the state of the shared buffer cache.
 </para>

 <para>
  The <function>pg_buffercache_usage_counts()</function> function returns a set
  of records, each row describing the number of buffers with a given usage
  count.
 </para>

 <para>
  By default, use is restricted to superusers and roles with privileges of the
  <literal>pg_monitor</literal> role. Access may be granted to others
  using <command>GRANT</command>.
 </para>

 <sect2 id="pgbuffercache-pg-buffercache">
  <title>The <structname>pg_buffercache</structname> View</title>

  <para>
   The definitions of the columns exposed by the view are shown in <xref linkend="pgbuffercache-columns"/>.
  </para>

  <table id="pgbuffercache-columns">
   <title><structname>pg_buffercache</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>bufferid</structfield> <type>integer</type>
      </para>
      <para>
       ID, in the range 1..<varname>shared_buffers</varname>
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>relfilenode</structfield> <type>oid</type>
       (references <link linkend="catalog-pg-class"><structname>pg_class</structname></link>.<structfield>relfilenode</structfield>)
      </para>
      <para>
       Filenode number of the relation
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>reltablespace</structfield> <type>oid</type>
       (references <link linkend="catalog-pg-tablespace"><structname>pg_tablespace</structname></link>.<structfield>oid</structfield>)
      </para>
      <para>
       Tablespace OID of the relation
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>reldatabase</structfield> <type>oid</type>
       (references <link linkend="catalog-pg-database"><structname>pg_database</structname></link>.<structfield>oid</structfield>)
      </para>
      <para>
       Database OID of the relation
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>relforknumber</structfield> <type>smallint</type>
      </para>
      <para>
       Fork number within the relation;  see
       <filename>common/relpath.h</filename>
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>relblocknumber</structfield> <type>bigint</type>
      </para>
      <para>
       Page number within the relation
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>isdirty</structfield> <type>boolean</type>
      </para>
      <para>
       Is the page dirty?
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>usagecount</structfield> <type>smallint</type>
      </para>
      <para>
       Clock-sweep access count
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>pinning_backends</structfield> <type>integer</type>
      </para>
      <para>
       Number of backends pinning this buffer
      </para></entry>
     </row>
    </tbody>
   </tgroup>
  </table>

  <para>
   There is one row for each buffer in the shared cache. Unused buffers are
   shown with all fields null except <structfield>bufferid</structfield>.  Shared system
   catalogs are shown as belonging to database zero.
  </para>

  <para>
   Because the cache is shared by all the databases, there will normally be
   pages from relations not belonging to the current database.  This means
   that there may not be matching join rows in <structname>pg_class</structname> for
   some rows, or that there could even be incorrect joins.  If you are
   trying to join against <structname>pg_class</structname>, it's a good idea to
   restrict the join to rows having <structfield>reldatabase</structfield> equal to
   the current database's OID or zero.
  </para>

  <para>
   Since buffer manager locks are not taken to copy the buffer state data that
   the view will display, accessing <structname>pg_buffercache</structname> view
   has less impact on normal buffer activity but it doesn't provide a consistent
   set of results across all buffers.  However, we ensure that the information of
   each buffer is self-consistent.
  </para>
 </sect2>

 <sect2 id="pgbuffercache-summary">
  <title>The <function>pg_buffercache_summary()</function> Function</title>

  <para>
   The definitions of the columns exposed by the function are shown in <xref linkend="pgbuffercache-summary-columns"/>.
  </para>

  <table id="pgbuffercache-summary-columns">
   <title><function>pg_buffercache_summary()</function> Output 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>buffers_used</structfield> <type>int4</type>
      </para>
      <para>
       Number of used shared buffers
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>buffers_unused</structfield> <type>int4</type>
      </para>
      <para>
       Number of unused shared buffers
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>buffers_dirty</structfield> <type>int4</type>
      </para>
      <para>
       Number of dirty shared buffers
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>buffers_pinned</structfield> <type>int4</type>
      </para>
      <para>
       Number of pinned shared buffers
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>usagecount_avg</structfield> <type>float8</type>
      </para>
      <para>
       Average usage count of used shared buffers
      </para></entry>
     </row>
    </tbody>
   </tgroup>
  </table>

  <para>
   The <function>pg_buffercache_summary()</function> function returns a
   single row summarizing the state of all shared buffers. Similar and more
   detailed information is provided by the
   <structname>pg_buffercache</structname> view, but
   <function>pg_buffercache_summary()</function> is significantly cheaper.
  </para>

  <para>
   Like the <structname>pg_buffercache</structname> view,
   <function>pg_buffercache_summary()</function> does not acquire buffer
   manager locks. Therefore concurrent activity can lead to minor inaccuracies
   in the result.
  </para>
 </sect2>

 <sect2 id="pgbuffercache-usage-counts">
  <title>The <function>pg_buffercache_usage_counts()</function> Function</title>

  <para>
   The definitions of the columns exposed by the function are shown in
   <xref linkend="pgbuffercache_usage_counts-columns"/>.
  </para>

  <table id="pgbuffercache_usage_counts-columns">
   <title><function>pg_buffercache_usage_counts()</function> Output 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>usage_count</structfield> <type>int4</type>
      </para>
      <para>
       A possible buffer usage count
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>buffers</structfield> <type>int4</type>
      </para>
      <para>
       Number of buffers with the usage count
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>dirty</structfield> <type>int4</type>
      </para>
      <para>
       Number of dirty buffers with the usage count
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>pinned</structfield> <type>int4</type>
      </para>
      <para>
       Number of pinned buffers with the usage count
      </para></entry>
     </row>
    </tbody>
   </tgroup>
  </table>

  <para>
   The <function>pg_buffercache_usage_counts()</function> function returns a
   set of rows summarizing the states of all shared buffers, aggregated over
   the possible usage count values.  Similar and more detailed information is
   provided by the <structname>pg_buffercache</structname> view, but
   <function>pg_buffercache_usage_counts()</function> is significantly cheaper.
  </para>

  <para>
   Like the <structname>pg_buffercache</structname> view,
   <function>pg_buffercache_usage_counts()</function> does not acquire buffer
   manager locks. Therefore concurrent activity can lead to minor inaccuracies
   in the result.
  </para>
 </sect2>

 <sect2 id="pgbuffercache-sample-output">
  <title>Sample Output</title>

<screen>
regression=# SELECT n.nspname, c.relname, count(*) AS buffers
             FROM pg_buffercache b JOIN pg_class c
             ON b.relfilenode = pg_relation_filenode(c.oid) AND
                b.reldatabase IN (0, (SELECT oid FROM pg_database
                                      WHERE datname = current_database()))
             JOIN pg_namespace n ON n.oid = c.relnamespace
             GROUP BY n.nspname, c.relname
             ORDER BY 3 DESC
             LIMIT 10;

  nspname   |        relname         | buffers
------------+------------------------+---------
 public     | delete_test_table      |     593
 public     | delete_test_table_pkey |     494
 pg_catalog | pg_attribute           |     472
 public     | quad_poly_tbl          |     353
 public     | tenk2                  |     349
 public     | tenk1                  |     349
 public     | gin_test_idx           |     306
 pg_catalog | pg_largeobject         |     206
 public     | gin_test_tbl           |     188
 public     | spgist_text_tbl        |     182
(10 rows)


regression=# SELECT * FROM pg_buffercache_summary();
 buffers_used | buffers_unused | buffers_dirty | buffers_pinned | usagecount_avg
--------------+----------------+---------------+----------------+----------------
          248 |        2096904 |            39 |              0 |       3.141129
(1 row)


regression=# SELECT * FROM pg_buffercache_usage_counts();
 usage_count | buffers | dirty | pinned
-------------+---------+-------+--------
           0 |   14650 |     0 |      0
           1 |    1436 |   671 |      0
           2 |     102 |    88 |      0
           3 |      23 |    21 |      0
           4 |       9 |     7 |      0
           5 |     164 |   106 |      0
(6 rows)
</screen>
 </sect2>

 <sect2 id="pgbuffercache-authors">
  <title>Authors</title>

  <para>
   Mark Kirkwood <email>markir@paradise.net.nz</email>
  </para>

  <para>
   Design suggestions: Neil Conway <email>neilc@samurai.com</email>
  </para>

  <para>
   Debugging advice: Tom Lane <email>tgl@sss.pgh.pa.us</email>
  </para>
 </sect2>

</sect1>