244 lines
8.5 KiB
Plaintext
244 lines
8.5 KiB
Plaintext
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<sect1 id="pgfreespacemap">
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<title>pgfreespacemap</title>
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<indexterm zone="pgfreespacemap">
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<primary>pgfreespacemap</primary>
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</indexterm>
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<para>
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This modules provides the means for examining the free space map (FSM). It
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consists of two C functions: <literal>pg_freespacemap_relations()</literal>
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and <literal>pg_freespacemap_pages()</literal> that return a set
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of records, plus two views <literal>pg_freespacemap_relations</literal> and
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<literal>pg_freespacemap_pages</literal> for more user-friendly access to
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the functions.
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</para>
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<para>
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The module provides the ability to examine the contents of the free space
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map, without having to restart or rebuild the server with additional
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debugging code.
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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 present in the code.
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</para>
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<sect2>
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<title>Notes</title>
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<para>
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The definitions for the columns exposed in the views are:
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</para>
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<table>
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<title>pg_freespacemap_relations</title>
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<tgroup cols="3">
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<thead>
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<row>
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<entry>Column</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>reltablespace</entry>
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<entry>pg_tablespace.oid</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>reldatabase</entry>
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<entry>pg_database.oid</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>relfilenode</entry>
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<entry>pg_class.relfilenode</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>avgrequest</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>interestingpages</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>storedpages</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>nextpage</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>pg_freespacemap_pages</title>
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<tgroup cols="3">
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<thead>
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<row>
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<entry>Column</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>reltablespace</entry>
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<entry>pg_tablespace.oid</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>reldatabase</entry>
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<entry>pg_database.oid</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>relfilenode</entry>
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<entry>pg_class.relfilenode</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>relblocknumber</entry>
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<entry></entry>
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<entry>Page number in the relation.</entry>
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</row>
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<row>
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<entry>bytes</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 <literal>pg_freespacemap_relations</literal>, there is one row for each
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relation in the free space map. <literal>storedpages</literal> is the
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number of pages actually stored in the map, while
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<literal>interestingpages</literal> is the number of pages the last VACUUM
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thought had useful amounts of free space.
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</para>
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<para>
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If <literal>storedpages</literal> is consistently less than interestingpages
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then it'd be a good idea to increase <literal>max_fsm_pages</literal>. Also,
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if the number of rows in <literal>pg_freespacemap_relations</literal> is
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close to <literal>max_fsm_relations</literal>, then you should consider
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increasing <literal>max_fsm_relations</literal>.
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</para>
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<para>
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For <literal>pg_freespacemap_pages</literal>, there is one row for each page
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in the free space map. The number of rows for a relation will match the
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<literal>storedpages</literal> column in
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<literal>pg_freespacemap_relations</literal>.
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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, it will include relations
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not belonging to the current database.
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</para>
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<para>
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When either of the views are accessed, internal free space map locks are
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taken, and a copy of the map data is made for them to 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 - pg_freespacemap_relations</title>
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<programlisting>
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regression=# \d pg_freespacemap_relations
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View "public.pg_freespacemap_relations"
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Column | Type | Modifiers
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------------------+---------+-----------
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reltablespace | oid |
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reldatabase | oid |
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relfilenode | oid |
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avgrequest | integer |
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interestingpages | integer |
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storedpages | integer |
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nextpage | integer |
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View definition:
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SELECT p.reltablespace, p.reldatabase, p.relfilenode, p.avgrequest, p.interestingpages, p.storedpages, p.nextpage
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FROM pg_freespacemap_relations() p(reltablespace oid, reldatabase oid, relfilenode oid, avgrequest integer, interestingpages integer, storedpages integer, nextpage integer);
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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 c.relfilenode = r.relfilenode INNER JOIN pg_database d
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ON r.reldatabase = d.oid AND (d.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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</programlisting>
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</sect2>
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<sect2>
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<title>Sample output - pg_freespacemap_pages</title>
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<programlisting>
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regression=# \d pg_freespacemap_pages
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View "public.pg_freespacemap_pages"
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Column | Type | Modifiers
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----------------+---------+-----------
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reltablespace | oid |
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reldatabase | oid |
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relfilenode | oid |
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relblocknumber | bigint |
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bytes | integer |
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View definition:
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SELECT p.reltablespace, p.reldatabase, p.relfilenode, p.relblocknumber, p.bytes
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FROM pg_freespacemap_pages() p(reltablespace oid, reldatabase oid, relfilenode oid, relblocknumber bigint, bytes integer);
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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 c.relfilenode = p.relfilenode INNER JOIN pg_database d
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ON (p.reldatabase = d.oid AND d.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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