doc/src/sgml/pgfreespacemap.sgml - master - githubmirror/postgres

doc/src/sgml/pgfreespacemap.sgml
121 строка3 KB
Michael Paquier
doc: Correct description of values used in FSM for indexes
27 мар 2025, 04:20
27 мар 2025, 04:2044fe6ce
<!-- doc/src/sgml/pgfreespacemap.sgml -->

<sect1 id="pgfreespacemap" xreflabel="pg_freespacemap">
 <title>pg_freespacemap &mdash; examine the free space map</title>

 <indexterm zone="pgfreespacemap">
  <primary>pg_freespacemap</primary>
 </indexterm>

 <para>
  The <filename>pg_freespacemap</filename> module provides a means for examining the
  <link linkend="storage-fsm">free space map</link> (<acronym>FSM</acronym>).
  It provides a function called <function>pg_freespace</function>, or two
  overloaded functions, to be precise. The functions show the value recorded in
  the free space map for a given page, or for all pages in the relation.
 </para>

 <para>
  By default use is restricted to superusers and roles with privileges of the
  <literal>pg_stat_scan_tables</literal> role. Access may be granted to others
  using <command>GRANT</command>.
 </para>

 <sect2 id="pgfreespacemap-funcs">
  <title>Functions</title>

  <variablelist>
   <varlistentry>
    <term>
     <function>pg_freespace(rel regclass IN, blkno bigint IN) returns int2</function>
     <indexterm>
      <primary>pg_freespace</primary>
     </indexterm>
    </term>

    <listitem>
     <para>
      Returns the amount of free space on the page of the relation, specified
      by <literal>blkno</literal>, according to the <acronym>FSM</acronym>.
     </para>
    </listitem>
   </varlistentry>


   <varlistentry>
    <term>
     <function>pg_freespace(rel regclass IN, blkno OUT bigint, avail OUT int2)</function>
    </term>

    <listitem>
     <para>
      Displays the amount of free space on each page of the relation,
      according to the <acronym>FSM</acronym>. A set of
      <literal>(blkno bigint, avail int2)</literal>
      tuples is returned, one tuple for each page in the relation.
     </para>
    </listitem>
   </varlistentry>
  </variablelist>

  <para>
   The values stored in the free space map are not exact. They're rounded
   to precision of 1/256th of <symbol>BLCKSZ</symbol> (32 bytes with default <symbol>BLCKSZ</symbol>), and
   they're not kept fully up-to-date as tuples are inserted and updated.
  </para>

  <para>
   For indexes, what is tracked is entirely-unused pages, rather than free
   space within pages.  Therefore, the values are not meaningful, just
   whether a page is in-use or empty.
  </para>
 </sect2>

 <sect2 id="pgfreespacemap-sample-output">
  <title>Sample Output</title>

<screen>
postgres=# SELECT * FROM pg_freespace('foo');
 blkno | avail
-------+-------
     0 |     0
     1 |     0
     2 |     0
     3 |    32
     4 |   704
     5 |   704
     6 |   704
     7 |  1216
     8 |   704
     9 |   704
    10 |   704
    11 |   704
    12 |   704
    13 |   704
    14 |   704
    15 |   704
    16 |   704
    17 |   704
    18 |   704
    19 |  3648
(20 rows)

postgres=# SELECT * FROM pg_freespace('foo', 7);
 pg_freespace
--------------
         1216
(1 row)
</screen>
 </sect2>

 <sect2 id="pgfreespacemap-author">
  <title>Author</title>

  <para>
   Original version by Mark Kirkwood <email>markir@paradise.net.nz</email>.
   Rewritten in version 8.4 to suit new <acronym>FSM</acronym> implementation
   by Heikki Linnakangas <email>heikki@enterprisedb.com</email>
  </para>
 </sect2>

</sect1>