323 lines
10 KiB
Plaintext
323 lines
10 KiB
Plaintext
<!-- doc/src/sgml/pgbuffercache.sgml -->
|
|
|
|
<sect1 id="pgbuffercache" xreflabel="pg_buffercache">
|
|
<title>pg_buffercache — inspect <productname>PostgreSQL</productname>
|
|
buffer cache state</title>
|
|
|
|
<indexterm zone="pgbuffercache">
|
|
<primary>pg_buffercache</primary>
|
|
</indexterm>
|
|
|
|
<para>
|
|
The <filename>pg_buffercache</filename> module provides a means for
|
|
examining what's happening in the shared buffer cache in real time.
|
|
</para>
|
|
|
|
<indexterm>
|
|
<primary>pg_buffercache_pages</primary>
|
|
</indexterm>
|
|
|
|
<indexterm>
|
|
<primary>pg_buffercache_summary</primary>
|
|
</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.
|
|
</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
|
|
using <command>GRANT</command>.
|
|
</para>
|
|
|
|
<sect2 id="pgbuffercache-pg-buffercache">
|
|
<title>The <structname>pg_buffercache</structname> View</title>
|
|
|
|
<para>
|
|
The definitions of the columns exposed by the view are shown in <xref linkend="pgbuffercache-columns"/>.
|
|
</para>
|
|
|
|
<table id="pgbuffercache-columns">
|
|
<title><structname>pg_buffercache</structname> 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>bufferid</structfield> <type>integer</type>
|
|
</para>
|
|
<para>
|
|
ID, in the range 1..<varname>shared_buffers</varname>
|
|
</para></entry>
|
|
</row>
|
|
|
|
<row>
|
|
<entry role="catalog_table_entry"><para role="column_definition">
|
|
<structfield>relfilenode</structfield> <type>oid</type>
|
|
(references <link linkend="catalog-pg-class"><structname>pg_class</structname></link>.<structfield>relfilenode</structfield>)
|
|
</para>
|
|
<para>
|
|
Filenode number of the relation
|
|
</para></entry>
|
|
</row>
|
|
|
|
<row>
|
|
<entry role="catalog_table_entry"><para role="column_definition">
|
|
<structfield>reltablespace</structfield> <type>oid</type>
|
|
(references <link linkend="catalog-pg-tablespace"><structname>pg_tablespace</structname></link>.<structfield>oid</structfield>)
|
|
</para>
|
|
<para>
|
|
Tablespace OID of the relation
|
|
</para></entry>
|
|
</row>
|
|
|
|
<row>
|
|
<entry role="catalog_table_entry"><para role="column_definition">
|
|
<structfield>reldatabase</structfield> <type>oid</type>
|
|
(references <link linkend="catalog-pg-database"><structname>pg_database</structname></link>.<structfield>oid</structfield>)
|
|
</para>
|
|
<para>
|
|
Database OID of the relation
|
|
</para></entry>
|
|
</row>
|
|
|
|
<row>
|
|
<entry role="catalog_table_entry"><para role="column_definition">
|
|
<structfield>relforknumber</structfield> <type>smallint</type>
|
|
</para>
|
|
<para>
|
|
Fork number within the relation; see
|
|
<filename>common/relpath.h</filename>
|
|
</para></entry>
|
|
</row>
|
|
|
|
<row>
|
|
<entry role="catalog_table_entry"><para role="column_definition">
|
|
<structfield>relblocknumber</structfield> <type>bigint</type>
|
|
</para>
|
|
<para>
|
|
Page number within the relation
|
|
</para></entry>
|
|
</row>
|
|
|
|
<row>
|
|
<entry role="catalog_table_entry"><para role="column_definition">
|
|
<structfield>isdirty</structfield> <type>boolean</type>
|
|
</para>
|
|
<para>
|
|
Is the page dirty?
|
|
</para></entry>
|
|
</row>
|
|
|
|
<row>
|
|
<entry role="catalog_table_entry"><para role="column_definition">
|
|
<structfield>usagecount</structfield> <type>smallint</type>
|
|
</para>
|
|
<para>
|
|
Clock-sweep access count
|
|
</para></entry>
|
|
</row>
|
|
|
|
<row>
|
|
<entry role="catalog_table_entry"><para role="column_definition">
|
|
<structfield>pinning_backends</structfield> <type>integer</type>
|
|
</para>
|
|
<para>
|
|
Number of backends pinning this buffer
|
|
</para></entry>
|
|
</row>
|
|
</tbody>
|
|
</tgroup>
|
|
</table>
|
|
|
|
<para>
|
|
There is one row for each buffer in the shared cache. Unused buffers are
|
|
shown with all fields null except <structfield>bufferid</structfield>. Shared system
|
|
catalogs are shown as belonging to database zero.
|
|
</para>
|
|
|
|
<para>
|
|
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 <structname>pg_class</structname> for
|
|
some rows, or that there could even be incorrect joins. If you are
|
|
trying to join against <structname>pg_class</structname>, it's a good idea to
|
|
restrict the join to rows having <structfield>reldatabase</structfield> equal to
|
|
the current database's OID or zero.
|
|
</para>
|
|
|
|
<para>
|
|
Since buffer manager locks are not taken to copy the buffer state data that
|
|
the view will display, accessing <structname>pg_buffercache</structname> 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.
|
|
</para>
|
|
</sect2>
|
|
|
|
<sect2 id="pgbuffercache-summary">
|
|
<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 id="pgbuffercache-sample-output">
|
|
<title>Sample Output</title>
|
|
|
|
<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)
|
|
</screen>
|
|
</sect2>
|
|
|
|
<sect2 id="pgbuffercache-authors">
|
|
<title>Authors</title>
|
|
|
|
<para>
|
|
Mark Kirkwood <email>markir@paradise.net.nz</email>
|
|
</para>
|
|
|
|
<para>
|
|
Design suggestions: Neil Conway <email>neilc@samurai.com</email>
|
|
</para>
|
|
|
|
<para>
|
|
Debugging advice: Tom Lane <email>tgl@sss.pgh.pa.us</email>
|
|
</para>
|
|
</sect2>
|
|
|
|
</sect1>
|