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<!-- 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.
</para>
<para>
These routines return information about three different bits. 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.
</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.
</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_VISIBILE</> 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 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 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>
</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>
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