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ba356a397d
Clarify libpq.sgml's description of service file locations and semantics. Avoid use of backtick'ed pg_config calls to describe paths; that doesn't work on Windows, and even on Unix it's an idiom that not all readers may be instantly familiar with. Don't overspecify the locations of include files, instead writing only as much as you'd use in #include directives. The previous text in these places was incorrect for some installations, depending on where "postgresql" is in the install path. Our convention for referencing the user's home directory seems to be "~", so change the one place that spelled it "$HOME". install-windows.sgml follows the platform convention of spelling file paths with "\", so change the one place that used "/". Haiying Tang and Tom Lane Discussion: https://postgr.es/m/162149020918.26174.7150424047314144297@wrigleys.postgresql.org
214 lines
6.6 KiB
Plaintext
214 lines
6.6 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, use is restricted to superusers and members of the
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<literal>pg_monitor</literal> role. Access may be granted to others
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using <command>GRANT</command>.
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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</structname> Columns</title>
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<tgroup cols="1">
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<thead>
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<row>
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<entry role="catalog_table_entry"><para role="column_definition">
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Column Type
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</para>
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<para>
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Description
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</para></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 role="catalog_table_entry"><para role="column_definition">
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<structfield>bufferid</structfield> <type>integer</type>
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</para>
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<para>
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ID, in the range 1..<varname>shared_buffers</varname>
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</para></entry>
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</row>
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<row>
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<entry role="catalog_table_entry"><para role="column_definition">
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<structfield>relfilenode</structfield> <type>oid</type>
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(references <link linkend="catalog-pg-class"><structname>pg_class</structname></link>.<structfield>relfilenode</structfield>)
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</para>
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<para>
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Filenode number of the relation
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</para></entry>
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</row>
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<row>
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<entry role="catalog_table_entry"><para role="column_definition">
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<structfield>reltablespace</structfield> <type>oid</type>
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(references <link linkend="catalog-pg-tablespace"><structname>pg_tablespace</structname></link>.<structfield>oid</structfield>)
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</para>
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<para>
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Tablespace OID of the relation
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</para></entry>
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</row>
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<row>
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<entry role="catalog_table_entry"><para role="column_definition">
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<structfield>reldatabase</structfield> <type>oid</type>
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(references <link linkend="catalog-pg-database"><structname>pg_database</structname></link>.<structfield>oid</structfield>)
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</para>
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<para>
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Database OID of the relation
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</para></entry>
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</row>
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<row>
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<entry role="catalog_table_entry"><para role="column_definition">
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<structfield>relforknumber</structfield> <type>smallint</type>
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</para>
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<para>
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Fork number within the relation; see
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<filename>common/relpath.h</filename>
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</para></entry>
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</row>
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<row>
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<entry role="catalog_table_entry"><para role="column_definition">
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<structfield>relblocknumber</structfield> <type>bigint</type>
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</para>
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<para>
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Page number within the relation
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</para></entry>
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</row>
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<row>
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<entry role="catalog_table_entry"><para role="column_definition">
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<structfield>isdirty</structfield> <type>boolean</type>
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</para>
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<para>
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Is the page dirty?
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</para></entry>
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</row>
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<row>
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<entry role="catalog_table_entry"><para role="column_definition">
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<structfield>usagecount</structfield> <type>smallint</type>
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</para>
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<para>
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Clock-sweep access count
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</para></entry>
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</row>
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<row>
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<entry role="catalog_table_entry"><para role="column_definition">
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<structfield>pinning_backends</structfield> <type>integer</type>
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</para>
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<para>
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Number of backends pinning this buffer
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</para></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</structfield>. 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</structname> 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</structname>, it's a good idea to
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restrict the join to rows having <structfield>reldatabase</structfield> 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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Since buffer manager locks are not taken to copy the buffer state data that
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the view will display, accessing <structname>pg_buffercache</structname> view
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has less impact on normal buffer activity but it doesn't provide a consistent
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set of results across all buffers. However, we ensure that the information of
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each buffer is self-consistent.
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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 n.nspname, c.relname, count(*) AS buffers
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FROM pg_buffercache b 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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JOIN pg_namespace n ON n.oid = c.relnamespace
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GROUP BY n.nspname, c.relname
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ORDER BY 3 DESC
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LIMIT 10;
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nspname | relname | buffers
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------------+------------------------+---------
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public | delete_test_table | 593
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public | delete_test_table_pkey | 494
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pg_catalog | pg_attribute | 472
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public | quad_poly_tbl | 353
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public | tenk2 | 349
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public | tenk1 | 349
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public | gin_test_idx | 306
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pg_catalog | pg_largeobject | 206
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public | gin_test_tbl | 188
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public | spgist_text_tbl | 182
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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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