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185 lines
5.4 KiB
Plaintext
185 lines
5.4 KiB
Plaintext
<!-- doc/src/sgml/pgbuffercache.sgml -->
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<sect1 id="pgbuffercache" xreflabel="pg_buffercache">
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<title>pg_buffercache</title>
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<indexterm zone="pgbuffercache">
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<primary>pg_buffercache</primary>
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</indexterm>
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<para>
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The <filename>pg_buffercache</filename> module provides a means for
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examining what's happening in the shared buffer cache in real time.
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</para>
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<indexterm>
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<primary>pg_buffercache_pages</primary>
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</indexterm>
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<para>
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The module provides a C function <function>pg_buffercache_pages</function>
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that returns a set of records, plus a view
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<structname>pg_buffercache</structname> that wraps the function for
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convenient use.
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</para>
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<para>
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By default public access is revoked from both of these, just in case there
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are security issues lurking.
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</para>
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<sect2>
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<title>The <structname>pg_buffercache</structname> View</title>
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<para>
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The definitions of the columns exposed by the view are shown in <xref linkend="pgbuffercache-columns">.
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</para>
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<table id="pgbuffercache-columns">
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<title><structname>pg_buffercache</> Columns</title>
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<tgroup cols="4">
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<thead>
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<row>
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<entry>Name</entry>
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<entry>Type</entry>
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<entry>References</entry>
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<entry>Description</entry>
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</row>
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</thead>
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<tbody>
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<row>
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<entry><structfield>bufferid</structfield></entry>
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<entry><type>integer</type></entry>
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<entry></entry>
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<entry>ID, in the range 1..<varname>shared_buffers</></entry>
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</row>
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<row>
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<entry><structfield>relfilenode</structfield></entry>
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<entry><type>oid</type></entry>
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<entry><literal>pg_class.relfilenode</literal></entry>
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<entry>Filenode number of the relation</entry>
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</row>
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<row>
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<entry><structfield>reltablespace</structfield></entry>
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<entry><type>oid</type></entry>
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<entry><literal>pg_tablespace.oid</literal></entry>
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<entry>Tablespace OID of the relation</entry>
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</row>
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<row>
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<entry><structfield>reldatabase</structfield></entry>
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<entry><type>oid</type></entry>
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<entry><literal>pg_database.oid</literal></entry>
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<entry>Database OID of the relation</entry>
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</row>
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<row>
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<entry><structfield>relblocknumber</structfield></entry>
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<entry><type>bigint</type></entry>
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<entry></entry>
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<entry>Page number within the relation</entry>
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</row>
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<row>
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<entry><structfield>relforknumber</structfield></entry>
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<entry><type>smallint</type></entry>
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<entry></entry>
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<entry>Fork number within the relation; see
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<filename>include/storage/relfilenode.h</></entry>
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</row>
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<row>
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<entry><structfield>isdirty</structfield></entry>
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<entry><type>boolean</type></entry>
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<entry></entry>
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<entry>Is the page dirty?</entry>
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</row>
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<row>
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<entry><structfield>usagecount</structfield></entry>
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<entry><type>smallint</type></entry>
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<entry></entry>
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<entry>Clock-sweep access count</entry>
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</row>
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</tbody>
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</tgroup>
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</table>
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<para>
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There is one row for each buffer in the shared cache. Unused buffers are
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shown with all fields null except <structfield>bufferid</>. Shared system
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catalogs are shown as belonging to database zero.
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</para>
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<para>
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Because the cache is shared by all the databases, there will normally be
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pages from relations not belonging to the current database. This means
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that there may not be matching join rows in <structname>pg_class</> for
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some rows, or that there could even be incorrect joins. If you are
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trying to join against <structname>pg_class</>, it's a good idea to
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restrict the join to rows having <structfield>reldatabase</> equal to
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the current database's OID or zero.
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</para>
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<para>
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When the <structname>pg_buffercache</> view is accessed, internal buffer
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manager locks are taken for long enough to copy all the buffer state
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data that the view will display.
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This ensures that the view produces a consistent set of results, while not
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blocking normal buffer activity longer than necessary. Nonetheless there
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could be some impact on database performance if this view is read often.
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</para>
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</sect2>
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<sect2>
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<title>Sample Output</title>
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<screen>
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regression=# SELECT c.relname, count(*) AS buffers
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FROM pg_buffercache b INNER JOIN pg_class c
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ON b.relfilenode = pg_relation_filenode(c.oid) AND
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b.reldatabase IN (0, (SELECT oid FROM pg_database
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WHERE datname = current_database()))
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GROUP BY c.relname
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ORDER BY 2 DESC
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LIMIT 10;
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relname | buffers
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---------------------------------+---------
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tenk2 | 345
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tenk1 | 141
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pg_proc | 46
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pg_class | 45
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pg_attribute | 43
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pg_class_relname_nsp_index | 30
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pg_proc_proname_args_nsp_index | 28
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pg_attribute_relid_attnam_index | 26
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pg_depend | 22
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pg_depend_reference_index | 20
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(10 rows)
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</screen>
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</sect2>
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<sect2>
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<title>Authors</title>
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<para>
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Mark Kirkwood <email>markir@paradise.net.nz</email>
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</para>
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<para>
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Design suggestions: Neil Conway <email>neilc@samurai.com</email>
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</para>
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<para>
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Debugging advice: Tom Lane <email>tgl@sss.pgh.pa.us</email>
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</para>
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</sect2>
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</sect1>
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