247 lines
8.7 KiB
Plaintext
247 lines
8.7 KiB
Plaintext
<!-- $PostgreSQL: pgsql/doc/src/sgml/pgfreespacemap.sgml,v 2.2 2007/12/10 05:32:51 tgl Exp $ -->
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<sect1 id="pgfreespacemap">
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<title>pg_freespacemap</title>
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<indexterm zone="pgfreespacemap">
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<primary>pg_freespacemap</primary>
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</indexterm>
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<para>
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The <filename>pg_freespacemap</> module provides a means for examining the
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free space map (FSM). It provides two C functions:
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<function>pg_freespacemap_relations</function> and
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<function>pg_freespacemap_pages</function> that each return a set of
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records, plus two views <structname>pg_freespacemap_relations</structname>
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and <structname>pg_freespacemap_pages</structname> that wrap the functions
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for convenient use.
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</para>
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<para>
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By default public access is revoked from the functions and views, just in
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case there are security issues lurking.
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</para>
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<sect2>
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<title>The <filename>pg_freespacemap</> views</title>
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<para>
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The definitions of the columns exposed by the views are:
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</para>
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<table>
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<title><structname>pg_freespacemap_relations</> 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>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>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>Relfilenode of the relation</entry>
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</row>
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<row>
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<entry><structfield>avgrequest</structfield></entry>
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<entry><type>integer</type></entry>
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<entry></entry>
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<entry>Moving average of free space requests (NULL for indexes)</entry>
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</row>
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<row>
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<entry><structfield>interestingpages</structfield></entry>
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<entry><type>integer</type></entry>
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<entry></entry>
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<entry>Count of pages last reported as containing useful free space</entry>
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</row>
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<row>
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<entry><structfield>storedpages</structfield></entry>
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<entry><type>integer</type></entry>
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<entry></entry>
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<entry>Count of pages actually stored in free space map</entry>
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</row>
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<row>
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<entry><structfield>nextpage</structfield></entry>
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<entry><type>integer</type></entry>
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<entry></entry>
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<entry>Page index (from 0) to start next search at</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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<table>
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<title><structname>pg_freespacemap_pages</> 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>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>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>Relfilenode 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>bytes</structfield></entry>
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<entry><type>integer</type></entry>
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<entry></entry>
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<entry>Free bytes in the page, or NULL for an index page (see below)</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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For <structname>pg_freespacemap_relations</structname>, there is one row
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for each relation in the free space map.
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<structfield>storedpages</structfield> is the number of pages actually
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stored in the map, while <structfield>interestingpages</structfield> is the
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number of pages the last <command>VACUUM</> thought had useful amounts of
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free space.
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</para>
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<para>
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If <structfield>storedpages</structfield> is consistently less than
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<structfield>interestingpages</> then it'd be a good idea to increase
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<varname>max_fsm_pages</varname>. Also, if the number of rows in
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<structname>pg_freespacemap_relations</structname> is close to
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<varname>max_fsm_relations</varname>, then you should consider increasing
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<varname>max_fsm_relations</varname>.
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</para>
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<para>
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For <structname>pg_freespacemap_pages</structname>, there is one row for
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each page in the free space map. The number of rows for a relation will
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match the <structfield>storedpages</structfield> column in
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<structname>pg_freespacemap_relations</structname>.
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</para>
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<para>
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For indexes, what is tracked is entirely-unused pages, rather than free
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space within pages. Therefore, the average request size and free bytes
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within a page are not meaningful, and are shown as NULL.
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</para>
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<para>
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Because the map is shared by all the databases, there will normally be
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entries for 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 either of the views is accessed, internal free space map locks are
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taken for long enough to copy all the state data that the view will display.
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This ensures that the views produce a consistent set of results, while not
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blocking normal activity longer than necessary. Nonetheless there
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could be some impact on database performance if they are 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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<programlisting>
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regression=# SELECT c.relname, r.avgrequest, r.interestingpages, r.storedpages
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FROM pg_freespacemap_relations r INNER JOIN pg_class c
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ON r.relfilenode = c.relfilenode AND
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r.reldatabase IN (0, (SELECT oid FROM pg_database
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WHERE datname = current_database()))
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ORDER BY r.storedpages DESC LIMIT 10;
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relname | avgrequest | interestingpages | storedpages
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---------------------------------+------------+------------------+-------------
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onek | 256 | 109 | 109
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pg_attribute | 167 | 93 | 93
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pg_class | 191 | 49 | 49
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pg_attribute_relid_attnam_index | | 48 | 48
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onek2 | 256 | 37 | 37
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pg_depend | 95 | 26 | 26
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pg_type | 199 | 16 | 16
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pg_rewrite | 1011 | 13 | 13
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pg_class_relname_nsp_index | | 10 | 10
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pg_proc | 302 | 8 | 8
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(10 rows)
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regression=# SELECT c.relname, p.relblocknumber, p.bytes
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FROM pg_freespacemap_pages p INNER JOIN pg_class c
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ON p.relfilenode = c.relfilenode AND
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p.reldatabase IN (0, (SELECT oid FROM pg_database
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WHERE datname = current_database()))
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ORDER BY c.relname LIMIT 10;
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relname | relblocknumber | bytes
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--------------+----------------+-------
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a_star | 0 | 8040
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abstime_tbl | 0 | 7908
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aggtest | 0 | 8008
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altinhoid | 0 | 8128
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altstartwith | 0 | 8128
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arrtest | 0 | 7172
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b_star | 0 | 7976
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box_tbl | 0 | 7912
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bt_f8_heap | 54 | 7728
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bt_i4_heap | 49 | 8008
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(10 rows)
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</programlisting>
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</sect2>
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<sect2>
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<title>Author</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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</sect2>
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</sect1>
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