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pg_buffercache: Add pg_buffercache_summary()

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Using pg_buffercache_summary() is significantly cheaper than querying
pg_buffercache and summarizing in SQL.

Author: Melih Mutlu <m.melihmutlu@gmail.com>
Reviewed-by: Andres Freund <andres@anarazel.de>
Reviewed-by: Aleksander Alekseev <aleksander@timescale.com>
Reviewed-by: Zhang Mingli <zmlpostgres@gmail.com>
Discussion: https://postgr.es/m/CAGPVpCQAXYo54Q%3D8gqBsS%3Du0uk9qhnnq4%2B710BtUhUisX1XGEg%40mail.gmail.com
This commit is contained in:
Andres Freund
2022-10-13 09:55:46 -07:00
parent 7f8d9cedb3
commit 2589434ae0
8 changed files with 250 additions and 5 deletions
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114
doc/src/sgml/pgbuffercache.sgml
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@@ -16,13 +16,28 @@
<primary>pg_buffercache_pages</primary>
</indexterm>
<indexterm>
<primary>pg_buffercache_summary</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
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.
</para>
<para>
The <function>pg_buffercache_pages()</function> function returns a set of
records, each row describing the state of one shared buffer entry. The
<structname>pg_buffercache</structname> view wraps the function for
convenient use.
</para>
<para>
The <function>pg_buffercache_summary()</function> function returns a single
row summarizing the state of the shared buffer cache.
</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
@@ -164,6 +179,92 @@
</para>
</sect2>
<sect2>
<title>The <function>pg_buffercache_summary()</function> Function</title>
<para>
The definitions of the columns exposed by the function are shown in <xref linkend="pgbuffercache_summary-columns"/>.
</para>
<table id="pgbuffercache_summary-columns">
<title><function>pg_buffercache_summary()</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>buffers_used</structfield> <type>int4</type>
</para>
<para>
Number of unused shared buffers
</para></entry>
</row>
<row>
<entry role="catalog_table_entry"><para role="column_definition">
<structfield>buffers_unused</structfield> <type>int4</type>
</para>
<para>
Number of unused shared buffers
</para></entry>
</row>
<row>
<entry role="catalog_table_entry"><para role="column_definition">
<structfield>buffers_dirty</structfield> <type>int4</type>
</para>
<para>
Number of dirty shared buffers
</para></entry>
</row>
<row>
<entry role="catalog_table_entry"><para role="column_definition">
<structfield>buffers_pinned</structfield> <type>int4</type>
</para>
<para>
Number of pinned shared buffers
</para></entry>
</row>
<row>
<entry role="catalog_table_entry"><para role="column_definition">
<structfield>usagecount_avg</structfield> <type>float8</type>
</para>
<para>
Average usagecount of used shared buffers
</para></entry>
</row>
</tbody>
</tgroup>
</table>
<para>
The <function>pg_buffercache_summary()</function> function returns a
single row summarizing the state of all shared buffers. Similar and more
detailed information is provided by the
<structname>pg_buffercache</structname> view, but
<function>pg_buffercache_summary()</function> is significantly cheaper.
</para>
<para>
Like the <structname>pg_buffercache</structname> view,
<function>pg_buffercache_summary()</function> does not acquire buffer
manager locks. Therefore concurrent activity can lead to minor inaccuracies
in the result.
</para>
</sect2>
<sect2>
<title>Sample Output</title>
@@ -191,6 +292,13 @@ regression=# SELECT n.nspname, c.relname, count(*) AS buffers
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)
</screen>
</sect2>