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.
  It also offers a low-level way to evict data from it, for testing
  purposes.
 </para>

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

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

 <indexterm>
  <primary>pg_buffercache_evict</primary>
 </indexterm>

 <indexterm>
  <primary>pg_buffercache_evict_relation</primary>
 </indexterm>

 <indexterm>
  <primary>pg_buffercache_evict_all</primary>
 </indexterm>

 <para>
  This module provides the <function>pg_buffercache_pages()</function>
  function (wrapped in the <structname>pg_buffercache</structname> view),
  <function>pg_buffercache_numa_pages()</function> function (wrapped in the
  <structname>pg_buffercache_numa</structname> view), the
  <function>pg_buffercache_summary()</function> function, the
  <function>pg_buffercache_usage_counts()</function> function, the
  <function>pg_buffercache_evict()</function>, the
  <function>pg_buffercache_evict_relation()</function> function and the
  <function>pg_buffercache_evict_all()</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_numa_pages()</function> provides
  <acronym>NUMA</acronym> node mappings for shared buffer entries. This
  information is not part of <function>pg_buffercache_pages()</function>
  itself, as it is much slower to retrieve.
  The <structname>pg_buffercache_numa</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 of the above functions 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>

 <para>
  The <function>pg_buffercache_evict()</function> function allows a block to
  be evicted from the buffer pool given a buffer identifier.  Use of this
  function is restricted to superusers only.
 </para>

 <para>
  The <function>pg_buffercache_evict_relation()</function> function allows all
  unpinned shared buffers in the relation to be evicted from the buffer pool
  given a relation identifier.  Use of this function is restricted to
  superusers only.
 </para>

 <para>
  The <function>pg_buffercache_evict_all()</function> function allows all
  unpinned shared buffers to be evicted in the buffer pool.  Use of this
  function is restricted to superusers only.
 </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-pg-buffercache-numa">
  <title>The <structname>pg_buffercache_numa</structname> View</title>

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

  <table id="pgbuffercache-numa-columns">
   <title><structname>pg_buffercache_numa</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>os_page_num</structfield> <type>bigint</type>
      </para>
      <para>
       number of OS memory page for this buffer
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>numa_node</structfield> <type>int</type>
      </para>
      <para>
       ID of <acronym>NUMA</acronym> node
      </para></entry>
     </row>

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

  <para>
   As <acronym>NUMA</acronym> node ID inquiry for each page requires memory pages
   to be paged-in, the first execution of this function can take a noticeable
   amount of time. In all the cases (first execution or not), retrieving this
   information is costly and querying the view at a high frequency is not recommended.
  </para>

  <warning>
    <para>
      When determining the <acronym>NUMA</acronym> node, the view touches
      all memory pages for the shared memory segment. This will force
      allocation of the shared memory, if it wasn't allocated already,
      and the memory may get allocated in a single <acronym>NUMA</acronym>
      node (depending on system configuration).
    </para>
  </warning>

 </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-pg-buffercache-evict">
  <title>The <function>pg_buffercache_evict()</function> Function</title>
  <para>
   The <function>pg_buffercache_evict()</function> function takes a buffer
   identifier, as shown in the <structfield>bufferid</structfield> column of
   the <structname>pg_buffercache</structname> view.  It returns information
   about whether the buffer was evicted and flushed.  The buffer_evicted
   column is true on success, and false if the buffer wasn't valid, if it
   couldn't be evicted because it was pinned, or if it became dirty again
   after an attempt to write it out.  The buffer_flushed column is true if the
   buffer was flushed.  This does not necessarily mean that buffer was flushed
   by us, it might be flushed by someone else.  The result is immediately out
   of date upon return, as the buffer might become valid again at any time due
   to concurrent activity. The function is intended for developer testing
   only.
  </para>
 </sect2>

 <sect2 id="pgbuffercache-pg-buffercache-evict-relation">
  <title>The <structname>pg_buffercache_evict_relation</structname> Function</title>
  <para>
   The <function>pg_buffercache_evict_relation()</function> function is very
   similar to the <function>pg_buffercache_evict()</function> function.  The
   difference is that the <function>pg_buffercache_evict_relation()</function>
   takes a relation identifier instead of buffer identifier.  It tries to
   evict all buffers for all forks in that relation.

   It returns the number of evicted buffers, flushed buffers and the number of
   buffers that could not be evicted.  Flushed buffers haven't necessarily
   been flushed by us, they might have been flushed by someone else.  The
   result is immediately out of date upon return, as buffers might immediately
   be read back in due to concurrent activity.  The function is intended for
   developer testing only.
  </para>
 </sect2>

 <sect2 id="pgbuffercache-pg-buffercache-evict-all">
  <title>The <structname>pg_buffercache_evict_all</structname> Function</title>
  <para>
   The <function>pg_buffercache_evict_all()</function> function is very
   similar to the <function>pg_buffercache_evict()</function> function.  The
   difference is, the <function>pg_buffercache_evict_all()</function> function
   does not take an argument; instead it tries to evict all buffers in the
   buffer pool.  It returns the number of evicted buffers, flushed buffers and
   the number of buffers that could not be evicted.  Flushed buffers haven't
   necessarily been flushed by us, they might have been flushed by someone
   else.  The result is immediately out of date upon return, as buffers might
   immediately be read back in due to concurrent activity.  The function is
   intended for developer testing only.
  </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>