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<!-- 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 members 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>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>
<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>
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