<!-- doc/src/sgml/pgvisibility.sgml -->

<sect1 id="pgvisibility" xreflabel="pg_visibility">
 <title>pg_visibility</title>

 <indexterm zone="pgvisibility">
  <primary>pg_visibility</primary>
 </indexterm>

 <para>
  The <filename>pg_visibility</> module provides a means for examining the
  visibility map (VM) and page-level visibility information.  It also
  provides functions to check the integrity of the visibility map and to
  force it to be rebuilt.
 </para>

 <para>
  Three different bits are used to store information about page-level
  visibility.  The all-visible bit in the visibility map indicates that every
  tuple on a given page of a relation is visible to every current transaction.
  The all-frozen bit in the visibility map indicates that every tuple on the
  page is frozen; that is, no future vacuum will need to modify the page
  until such time as a tuple is inserted, updated, deleted, or locked on
  that page.  The page-level <literal>PD_ALL_VISIBLE</literal> bit has the
  same meaning as the all-visible bit in the visibility map, but is stored
  within the data page itself rather than a separate data structure.  These
  will normally agree, but the page-level bit can sometimes be set while the
  visibility map bit is clear after a crash recovery; or they can disagree
  because of a change which occurs after <literal>pg_visibility</> examines
  the visibility map and before it examines the data page.  Any event which
  causes data corruption can also cause these bits to disagree.
 </para>

 <para>
  Functions which display information about <literal>PD_ALL_VISIBLE</>
  are much more costly than those which only consult the visibility map,
  because they must read the relation's data blocks rather than only the
  (much smaller) visibility map.  Functions that check the relation's
  data blocks are similarly expensive.
 </para>

 <sect2>
  <title>Functions</title>

  <variablelist>
   <varlistentry>
    <term><function>pg_visibility_map(regclass, blkno bigint, all_visible OUT boolean, all_frozen OUT boolean) returns record</function></term>
    <listitem>
     <para>
      Returns the all-visible and all-frozen bits in the visibility map for
      the given block of the given relation.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><function>pg_visibility(regclass, blkno bigint, all_visible OUT boolean, all_frozen OUT boolean, pd_all_visible OUT boolean) returns record</function></term>
    <listitem>
     <para>
      Returns the all-visible and all-frozen bits in the visibility map for
      the given block of the given relation, plus the
      <literal>PD_ALL_VISIBLE</> bit for that block.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><function>pg_visibility_map(regclass, blkno OUT bigint, all_visible OUT boolean, all_frozen OUT boolean) returns setof record</function></term>
    <listitem>
     <para>
      Returns the all-visible and all-frozen bits in the visibility map for
      each block the given relation.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><function>pg_visibility(regclass, blkno OUT bigint, all_visible OUT boolean, all_frozen OUT boolean, pd_all_visible OUT boolean) returns setof record</function></term>

    <listitem>
     <para>
      Returns the all-visible and all-frozen bits in the visibility map for
      each block the given relation, plus the <literal>PD_ALL_VISIBLE</>
      bit for each block.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><function>pg_visibility_map_summary(regclass, all_visible OUT bigint, all_frozen OUT bigint) returns record</function></term>

    <listitem>
     <para>
      Returns the number of all-visible pages and the number of all-frozen
      pages in the relation according to the visibility map.
     </para>
    </listitem>
   </varlistentry>
  
   <varlistentry>
    <term><function>pg_check_frozen(regclass, t_ctid OUT tid) returns setof tid</function></term>

    <listitem>
     <para>
      Returns the TIDs of non-frozen tuples present in pages marked all-frozen
      in the visibility map.  If this function returns a non-empty set of
      TIDs, the database is corrupt.
     </para>
    </listitem>
   </varlistentry>
     
    <varlistentry>
    <term><function>pg_check_visible(regclass, t_ctid OUT tid) returns setof tid</function></term>

    <listitem>
     <para>
      Returns the TIDs of tuples which are not all-visible despite the fact
      that the pages which contain them are marked as all-visible in the
      visibility map.  If this function returns a non-empty set of TIDs, the
      database is corrupt.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><function>pg_truncate_visibility_map(regclass) returns void</function></term>

    <listitem>
     <para>
      Truncates the visibility map for the given relation.  This function
      is only expected to be useful if you suspect that the visibility map
      for the indicated relation is corrupt and wish to rebuild it.  The first
      <command>VACUUM</> executed on the given relation after this function
      is executed will scan every page in the relation and rebuild the
      visibility map.
     </para>
    </listitem>
   </varlistentry>
  </variablelist>

  <para>
   By default, these functions are not publicly executable.
  </para>
 </sect2>

 <sect2>
  <title>Author</title>

  <para>
   Robert Haas <email>rhaas@postgresql.org</email>
  </para>
 </sect2>

</sect1>
