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Add pg_buffercache_usage_counts() to contrib/pg_buffercache.

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It was pointed out that pg_buffercache_summary()'s report of
the overall average usage count isn't that useful, and what
would be more helpful in many cases is to report totals for
each possible usage count.  Add a new function to do it like
that.  Since pg_buffercache 1.4 is already new for v16,
we don't need to create a new extension version; we'll just
define this as part of 1.4.

Nathan Bossart

Discussion: https://postgr.es/m/20230130233040.GA2800702@nathanxps13
This commit is contained in:
Tom Lane
2023-04-07 14:25:45 -04:00
parent e056c557ae
commit f3fa31327e
5 changed files with 173 additions and 4 deletions
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105
doc/src/sgml/pgbuffercache.sgml
View File

@ -22,9 +22,10 @@
</indexterm>
<para>
The module provides the <function>pg_buffercache_pages()</function>
function, wrapped in the <structname>pg_buffercache</structname> view, and
the <function>pg_buffercache_summary()</function> function.
This module provides the <function>pg_buffercache_pages()</function>
function (wrapped in the <structname>pg_buffercache</structname> view),
the <function>pg_buffercache_summary()</function> function, and the
<function>pg_buffercache_usage_counts()</function> function.
</para>
<para>
@ -39,6 +40,12 @@
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 is restricted to superusers and roles with privileges of the
<literal>pg_monitor</literal> role. Access may be granted to others
@ -243,7 +250,7 @@
<structfield>usagecount_avg</structfield> <type>float8</type>
</para>
<para>
Average usagecount of used shared buffers
Average usage count of used shared buffers
</para></entry>
</row>
</tbody>
@ -266,6 +273,84 @@
</para>
</sect2>
<sect2>
<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-sample-output">
<title>Sample Output</title>
@ -300,6 +385,18 @@ regression=# SELECT * FROM pg_buffercache_summary();
--------------+----------------+---------------+----------------+----------------
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>