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<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<!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.27. 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.26. passwordcheck" /><link rel="next" href="pgcrypto.html" title="F.28. pgcrypto" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">F.27. pg_buffercache</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="passwordcheck.html" title="F.26. 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 15.5 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="pgcrypto.html" title="F.28. pgcrypto">Next</a></td></tr></table><hr /></div><div class="sect1" id="PGBUFFERCACHE"><div class="titlepage"><div><div><h2 class="title" style="clear: both">F.27. pg_buffercache</h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="sect2"><a href="pgbuffercache.html#id-1.11.7.36.7">F.27.1. The <code class="structname">pg_buffercache</code> View</a></span></dt><dt><span class="sect2"><a href="pgbuffercache.html#id-1.11.7.36.8">F.27.2. Sample Output</a></span></dt><dt><span class="sect2"><a href="pgbuffercache.html#id-1.11.7.36.9">F.27.3. Authors</a></span></dt></dl></div><a id="id-1.11.7.36.2" 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-1.11.7.36.4" 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 roles with privileges of the
<code class="literal">pg_monitor</code> role. Access may be granted to others
using <code class="command">GRANT</code>.
</p><div class="sect2" id="id-1.11.7.36.7"><div class="titlepage"><div><div><h3 class="title">F.27.1. 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 /></colgroup><thead><tr><th class="catalog_table_entry"><p class="column_definition">
Column Type
</p>
<p>
Description
</p></th></tr></thead><tbody><tr><td class="catalog_table_entry"><p class="column_definition">
<code class="structfield">bufferid</code> <code class="type">integer</code>
</p>
<p>
ID, in the range 1..<code class="varname">shared_buffers</code>
</p></td></tr><tr><td class="catalog_table_entry"><p class="column_definition">
<code class="structfield">relfilenode</code> <code class="type">oid</code>
(references <a class="link" href="catalog-pg-class.html" title="53.11. pg_class"><code class="structname">pg_class</code></a>.<code class="structfield">relfilenode</code>)
</p>
<p>
Filenode number of the relation
</p></td></tr><tr><td class="catalog_table_entry"><p class="column_definition">
<code class="structfield">reltablespace</code> <code class="type">oid</code>
(references <a class="link" href="catalog-pg-tablespace.html" title="53.56. pg_tablespace"><code class="structname">pg_tablespace</code></a>.<code class="structfield">oid</code>)
</p>
<p>
Tablespace OID of the relation
</p></td></tr><tr><td class="catalog_table_entry"><p class="column_definition">
<code class="structfield">reldatabase</code> <code class="type">oid</code>
(references <a class="link" href="catalog-pg-database.html" title="53.15. pg_database"><code class="structname">pg_database</code></a>.<code class="structfield">oid</code>)
</p>
<p>
Database OID of the relation
</p></td></tr><tr><td class="catalog_table_entry"><p class="column_definition">
<code class="structfield">relforknumber</code> <code class="type">smallint</code>
</p>
<p>
Fork number within the relation; see
<code class="filename">common/relpath.h</code>
</p></td></tr><tr><td class="catalog_table_entry"><p class="column_definition">
<code class="structfield">relblocknumber</code> <code class="type">bigint</code>
</p>
<p>
Page number within the relation
</p></td></tr><tr><td class="catalog_table_entry"><p class="column_definition">
<code class="structfield">isdirty</code> <code class="type">boolean</code>
</p>
<p>
Is the page dirty?
</p></td></tr><tr><td class="catalog_table_entry"><p class="column_definition">
<code class="structfield">usagecount</code> <code class="type">smallint</code>
</p>
<p>
Clock-sweep access count
</p></td></tr><tr><td class="catalog_table_entry"><p class="column_definition">
<code class="structfield">pinning_backends</code> <code class="type">integer</code>
</p>
<p>
Number of backends pinning this buffer
</p></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>
Since buffer manager locks are not taken to copy the buffer state data that
the view will display, accessing <code class="structname">pg_buffercache</code> 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.
</p></div><div class="sect2" id="id-1.11.7.36.8"><div class="titlepage"><div><div><h3 class="title">F.27.2. Sample Output</h3></div></div></div><pre class="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)
</pre></div><div class="sect2" id="id-1.11.7.36.9"><div class="titlepage"><div><div><h3 class="title">F.27.3. 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>
</p></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="passwordcheck.html" title="F.26. passwordcheck">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="contrib.html" title="Appendix F. Additional Supplied Modules">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="pgcrypto.html" title="F.28. pgcrypto">Next</a></td></tr><tr><td width="40%" align="left" valign="top">F.26. passwordcheck </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 15.5 Documentation">Home</a></td><td width="40%" align="right" valign="top"> F.28. pgcrypto</td></tr></table></div></body></html>
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