postgres/doc/src/sgml/pgbuffercache.sgml

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

<sect1 id="pgbuffercache" xreflabel="pg_buffercache">
 <title>pg_buffercache</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>

 <para>
  The module provides a C function <function>pg_buffercache_pages</function>
  that returns a set of records, plus a view
  <structname>pg_buffercache</structname> that wraps the function for
  convenient use.
 </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>
  <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>int8</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>
  <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)
</screen>
 </sect2>

 <sect2>
  <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>