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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>F.24. pg_buffercache</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><link rel="prev" href="passwordcheck.html" title="F.23. passwordcheck" /><link rel="next" href="pgcrypto.html" title="F.25. pgcrypto" /></head><body><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">F.24. pg_buffercache</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="passwordcheck.html" title="F.23. passwordcheck">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="contrib.html" title="Appendix F. Additional Supplied Modules">Up</a></td><th width="60%" align="center">Appendix F. Additional Supplied Modules</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 11.5 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="pgcrypto.html" title="F.25. pgcrypto">Next</a></td></tr></table><hr></hr></div><div class="sect1" id="PGBUFFERCACHE"><div class="titlepage"><div><div><h2 class="title" style="clear: both">F.24. pg_buffercache</h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="sect2"><a href="pgbuffercache.html#id-1.11.7.33.7">F.24.1. The <code class="structname">pg_buffercache</code> View</a></span></dt><dt><span class="sect2"><a href="pgbuffercache.html#id-1.11.7.33.8">F.24.2. Sample Output</a></span></dt><dt><span class="sect2"><a href="pgbuffercache.html#id-1.11.7.33.9">F.24.3. Authors</a></span></dt></dl></div><a id="id-1.11.7.33.2" class="indexterm"></a><p>
The <code class="filename">pg_buffercache</code> module provides a means for
examining what's happening in the shared buffer cache in real time.
</p><a id="id-1.11.7.33.4" class="indexterm"></a><p>
The module provides a C function <code class="function">pg_buffercache_pages</code>
that returns a set of records, plus a view
<code class="structname">pg_buffercache</code> that wraps the function for
convenient use.
</p><p>
By default use is restricted to superusers and members of the
<code class="literal">pg_read_all_stats</code> role. Access may be granted to others
using <code class="command">GRANT</code>.
</p><div class="sect2" id="id-1.11.7.33.7"><div class="titlepage"><div><div><h3 class="title">F.24.1. The <code class="structname">pg_buffercache</code> View</h3></div></div></div><p>
The definitions of the columns exposed by the view are shown in <a class="xref" href="pgbuffercache.html#PGBUFFERCACHE-COLUMNS" title="Table F.15. pg_buffercache Columns">Table F.15</a>.
</p><div class="table" id="PGBUFFERCACHE-COLUMNS"><p class="title"><strong>Table F.15. <code class="structname">pg_buffercache</code> Columns</strong></p><div class="table-contents"><table class="table" summary="pg_buffercache Columns" border="1"><colgroup><col /><col /><col /><col /></colgroup><thead><tr><th>Name</th><th>Type</th><th>References</th><th>Description</th></tr></thead><tbody><tr><td><code class="structfield">bufferid</code></td><td><code class="type">integer</code></td><td> </td><td>ID, in the range 1..<code class="varname">shared_buffers</code></td></tr><tr><td><code class="structfield">relfilenode</code></td><td><code class="type">oid</code></td><td><code class="literal">pg_class.relfilenode</code></td><td>Filenode number of the relation</td></tr><tr><td><code class="structfield">reltablespace</code></td><td><code class="type">oid</code></td><td><code class="literal">pg_tablespace.oid</code></td><td>Tablespace OID of the relation</td></tr><tr><td><code class="structfield">reldatabase</code></td><td><code class="type">oid</code></td><td><code class="literal">pg_database.oid</code></td><td>Database OID of the relation</td></tr><tr><td><code class="structfield">relforknumber</code></td><td><code class="type">smallint</code></td><td> </td><td>Fork number within the relation; see
<code class="filename">include/common/relpath.h</code></td></tr><tr><td><code class="structfield">relblocknumber</code></td><td><code class="type">bigint</code></td><td> </td><td>Page number within the relation</td></tr><tr><td><code class="structfield">isdirty</code></td><td><code class="type">boolean</code></td><td> </td><td>Is the page dirty?</td></tr><tr><td><code class="structfield">usagecount</code></td><td><code class="type">smallint</code></td><td> </td><td>Clock-sweep access count</td></tr><tr><td><code class="structfield">pinning_backends</code></td><td><code class="type">integer</code></td><td> </td><td>Number of backends pinning this buffer</td></tr></tbody></table></div></div><br class="table-break" /><p>
There is one row for each buffer in the shared cache. Unused buffers are
shown with all fields null except <code class="structfield">bufferid</code>. Shared system
catalogs are shown as belonging to database zero.
</p><p>
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 <code class="structname">pg_class</code> for
some rows, or that there could even be incorrect joins. If you are
trying to join against <code class="structname">pg_class</code>, it's a good idea to
restrict the join to rows having <code class="structfield">reldatabase</code> equal to
the current database's OID or zero.
</p><p>
When the <code class="structname">pg_buffercache</code> view is accessed, internal buffer
manager locks are taken for long enough to copy all the buffer state
data that the view will display.
This ensures that the view produces a consistent set of results, while not
blocking normal buffer activity longer than necessary. Nonetheless there
could be some impact on database performance if this view is read often.
</p></div><div class="sect2" id="id-1.11.7.33.8"><div class="titlepage"><div><div><h3 class="title">F.24.2. Sample Output</h3></div></div></div><pre class="screen">
regression=# SELECT c.relname, count(*) AS buffers
FROM pg_buffercache b INNER 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()))
GROUP BY c.relname
ORDER BY 2 DESC
LIMIT 10;
relname | buffers
---------------------------------+---------
tenk2 | 345
tenk1 | 141
pg_proc | 46
pg_class | 45
pg_attribute | 43
pg_class_relname_nsp_index | 30
pg_proc_proname_args_nsp_index | 28
pg_attribute_relid_attnam_index | 26
pg_depend | 22
pg_depend_reference_index | 20
(10 rows)
</pre></div><div class="sect2" id="id-1.11.7.33.9"><div class="titlepage"><div><div><h3 class="title">F.24.3. Authors</h3></div></div></div><p>
Mark Kirkwood <code class="email"><<a class="email" href="mailto:markir@paradise.net.nz">markir@paradise.net.nz</a>></code>
</p><p>
Design suggestions: Neil Conway <code class="email"><<a class="email" href="mailto:neilc@samurai.com">neilc@samurai.com</a>></code>
</p><p>
Debugging advice: Tom Lane <code class="email"><<a class="email" href="mailto:tgl@sss.pgh.pa.us">tgl@sss.pgh.pa.us</a>></code>
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