128 lines
3.1 KiB
Plaintext
128 lines
3.1 KiB
Plaintext
<!-- doc/src/sgml/pgfreespacemap.sgml -->
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<sect1 id="pgfreespacemap" xreflabel="pg_freespacemap">
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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 a function called
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<function>pg_freespace</function>, or two overloaded functions, to be
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precise. The functions show the value recorded in the free space map for
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a given page, or for all pages in the relation.
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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_stat_scan_tables</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>Functions</title>
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<variablelist>
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<varlistentry>
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<term>
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<function>pg_freespace(rel regclass IN, blkno bigint IN) returns int2</function>
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<indexterm>
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<primary>pg_freespace</primary>
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</indexterm>
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</term>
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<listitem>
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<para>
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Returns the amount of free space on the page of the relation, specified
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by <literal>blkno</>, according to the FSM.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>
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<function>pg_freespace(rel regclass IN, blkno OUT bigint, avail OUT int2)</function>
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</term>
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<listitem>
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<para>
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Displays the amount of free space on each page of the relation,
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according to the FSM. A set of <literal>(blkno bigint, avail int2)</>
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tuples is returned, one tuple for each page in the relation.
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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<para>
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The values stored in the free space map are not exact. They're rounded
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to precision of 1/256th of <symbol>BLCKSZ</> (32 bytes with default <symbol>BLCKSZ</>), and
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they're not kept fully up-to-date as tuples are inserted and updated.
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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 values are not meaningful, just
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whether a page is full or empty.
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</para>
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<note>
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<para>
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The interface was changed in version 8.4, to reflect the new FSM
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implementation introduced in the same version.
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</para>
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</note>
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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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postgres=# SELECT * FROM pg_freespace('foo');
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blkno | avail
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-------+-------
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0 | 0
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1 | 0
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2 | 0
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3 | 32
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4 | 704
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5 | 704
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6 | 704
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7 | 1216
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8 | 704
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9 | 704
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10 | 704
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11 | 704
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12 | 704
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13 | 704
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14 | 704
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15 | 704
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16 | 704
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17 | 704
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18 | 704
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19 | 3648
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(20 rows)
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postgres=# SELECT * FROM pg_freespace('foo', 7);
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pg_freespace
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--------------
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1216
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(1 row)
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</screen>
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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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Original version by Mark Kirkwood <email>markir@paradise.net.nz</email>.
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Rewritten in version 8.4 to suit new FSM implementation by Heikki
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Linnakangas <email>heikki@enterprisedb.com</email>
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</para>
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</sect2>
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</sect1>
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