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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 — inspect PostgreSQL buffer cache state</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 — verify password strength" /><link rel="next" href="pgcrypto.html" title="F.28. pgcrypto — cryptographic functions" /></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 — inspect <span class="productname">PostgreSQL</span>
buffer cache state</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="passwordcheck.html" title="F.26. passwordcheck — verify password strength">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="contrib.html" title="Appendix F. Additional Supplied Modules and Extensions">Up</a></td><th width="60%" align="center">Appendix F. Additional Supplied Modules and Extensions</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 16.3 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="pgcrypto.html" title="F.28. pgcrypto — cryptographic functions">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 — inspect <span class="productname">PostgreSQL</span>
buffer cache state <a href="#PGBUFFERCACHE" class="id_link">#</a></h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="sect2"><a href="pgbuffercache.html#PGBUFFERCACHE-PG-BUFFERCACHE">F.27.1. The <code class="structname">pg_buffercache</code> View</a></span></dt><dt><span class="sect2"><a href="pgbuffercache.html#PGBUFFERCACHE-SUMMARY">F.27.2. The <code class="function">pg_buffercache_summary()</code> Function</a></span></dt><dt><span class="sect2"><a href="pgbuffercache.html#PGBUFFERCACHE-USAGE-COUNTS">F.27.3. The <code class="function">pg_buffercache_usage_counts()</code> Function</a></span></dt><dt><span class="sect2"><a href="pgbuffercache.html#PGBUFFERCACHE-SAMPLE-OUTPUT">F.27.4. Sample Output</a></span></dt><dt><span class="sect2"><a href="pgbuffercache.html#PGBUFFERCACHE-AUTHORS">F.27.5. Authors</a></span></dt></dl></div><a id="id-1.11.7.37.2" 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-1.11.7.37.4" class="indexterm"></a><a id="id-1.11.7.37.5" class="indexterm"></a><p>
This module provides the <code class="function">pg_buffercache_pages()</code>
function (wrapped in the <code class="structname">pg_buffercache</code> view),
the <code class="function">pg_buffercache_summary()</code> function, and the
<code class="function">pg_buffercache_usage_counts()</code> function.
</p><p>
The <code class="function">pg_buffercache_pages()</code> function returns a set of
records, each row describing the state of one shared buffer entry. The
<code class="structname">pg_buffercache</code> view wraps the function for
convenient use.
</p><p>
The <code class="function">pg_buffercache_summary()</code> function returns a single
row summarizing the state of the shared buffer cache.
</p><p>
The <code class="function">pg_buffercache_usage_counts()</code> function returns a set
of records, each row describing the number of buffers with a given usage
count.
</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="PGBUFFERCACHE-PG-BUFFERCACHE"><div class="titlepage"><div><div><h3 class="title">F.27.1. The <code class="structname">pg_buffercache</code> View <a href="#PGBUFFERCACHE-PG-BUFFERCACHE" class="id_link">#</a></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="PGBUFFERCACHE-SUMMARY"><div class="titlepage"><div><div><h3 class="title">F.27.2. The <code class="function">pg_buffercache_summary()</code> Function <a href="#PGBUFFERCACHE-SUMMARY" class="id_link">#</a></h3></div></div></div><p>
The definitions of the columns exposed by the function are shown in <a class="xref" href="pgbuffercache.html#PGBUFFERCACHE-SUMMARY-COLUMNS" title="Table F.16. pg_buffercache_summary() Output Columns">Table F.16</a>.
</p><div class="table" id="PGBUFFERCACHE-SUMMARY-COLUMNS"><p class="title"><strong>Table F.16. <code class="function">pg_buffercache_summary()</code> Output Columns</strong></p><div class="table-contents"><table class="table" summary="pg_buffercache_summary() Output 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">buffers_used</code> <code class="type">int4</code>
</p>
<p>
Number of used shared buffers
</p></td></tr><tr><td class="catalog_table_entry"><p class="column_definition">
<code class="structfield">buffers_unused</code> <code class="type">int4</code>
</p>
<p>
Number of unused shared buffers
</p></td></tr><tr><td class="catalog_table_entry"><p class="column_definition">
<code class="structfield">buffers_dirty</code> <code class="type">int4</code>
</p>
<p>
Number of dirty shared buffers
</p></td></tr><tr><td class="catalog_table_entry"><p class="column_definition">
<code class="structfield">buffers_pinned</code> <code class="type">int4</code>
</p>
<p>
Number of pinned shared buffers
</p></td></tr><tr><td class="catalog_table_entry"><p class="column_definition">
<code class="structfield">usagecount_avg</code> <code class="type">float8</code>
</p>
<p>
Average usage count of used shared buffers
</p></td></tr></tbody></table></div></div><br class="table-break" /><p>
The <code class="function">pg_buffercache_summary()</code> function returns a
single row summarizing the state of all shared buffers. Similar and more
detailed information is provided by the
<code class="structname">pg_buffercache</code> view, but
<code class="function">pg_buffercache_summary()</code> is significantly cheaper.
</p><p>
Like the <code class="structname">pg_buffercache</code> view,
<code class="function">pg_buffercache_summary()</code> does not acquire buffer
manager locks. Therefore concurrent activity can lead to minor inaccuracies
in the result.
</p></div><div class="sect2" id="PGBUFFERCACHE-USAGE-COUNTS"><div class="titlepage"><div><div><h3 class="title">F.27.3. The <code class="function">pg_buffercache_usage_counts()</code> Function <a href="#PGBUFFERCACHE-USAGE-COUNTS" class="id_link">#</a></h3></div></div></div><p>
The definitions of the columns exposed by the function are shown in
<a class="xref" href="pgbuffercache.html#PGBUFFERCACHE_USAGE_COUNTS-COLUMNS" title="Table F.17. pg_buffercache_usage_counts() Output Columns">Table F.17</a>.
</p><div class="table" id="PGBUFFERCACHE_USAGE_COUNTS-COLUMNS"><p class="title"><strong>Table F.17. <code class="function">pg_buffercache_usage_counts()</code> Output Columns</strong></p><div class="table-contents"><table class="table" summary="pg_buffercache_usage_counts() Output 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">usage_count</code> <code class="type">int4</code>
</p>
<p>
A possible buffer usage count
</p></td></tr><tr><td class="catalog_table_entry"><p class="column_definition">
<code class="structfield">buffers</code> <code class="type">int4</code>
</p>
<p>
Number of buffers with the usage count
</p></td></tr><tr><td class="catalog_table_entry"><p class="column_definition">
<code class="structfield">dirty</code> <code class="type">int4</code>
</p>
<p>
Number of dirty buffers with the usage count
</p></td></tr><tr><td class="catalog_table_entry"><p class="column_definition">
<code class="structfield">pinned</code> <code class="type">int4</code>
</p>
<p>
Number of pinned buffers with the usage count
</p></td></tr></tbody></table></div></div><br class="table-break" /><p>
The <code class="function">pg_buffercache_usage_counts()</code> 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 <code class="structname">pg_buffercache</code> view, but
<code class="function">pg_buffercache_usage_counts()</code> is significantly cheaper.
</p><p>
Like the <code class="structname">pg_buffercache</code> view,
<code class="function">pg_buffercache_usage_counts()</code> does not acquire buffer
manager locks. Therefore concurrent activity can lead to minor inaccuracies
in the result.
</p></div><div class="sect2" id="PGBUFFERCACHE-SAMPLE-OUTPUT"><div class="titlepage"><div><div><h3 class="title">F.27.4. Sample Output <a href="#PGBUFFERCACHE-SAMPLE-OUTPUT" class="id_link">#</a></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)
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)
</pre></div><div class="sect2" id="PGBUFFERCACHE-AUTHORS"><div class="titlepage"><div><div><h3 class="title">F.27.5. Authors <a href="#PGBUFFERCACHE-AUTHORS" class="id_link">#</a></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 — verify password strength">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="contrib.html" title="Appendix F. Additional Supplied Modules and Extensions">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="pgcrypto.html" title="F.28. pgcrypto — cryptographic functions">Next</a></td></tr><tr><td width="40%" align="left" valign="top">F.26. passwordcheck — verify password strength </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 16.3 Documentation">Home</a></td><td width="40%" align="right" valign="top"> F.28. pgcrypto — cryptographic functions</td></tr></table></div></body></html>
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