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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</filename> module provides a means for examining the
visibility map (VM) and page-level visibility information of a table.
It also provides functions to check the integrity of a 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 in the corresponding page of the relation is visible to every current
and future transaction. The all-frozen bit in the visibility map indicates
that every tuple in 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 header's <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 in a separate data structure.
These two bits will normally agree, but the page's all-visible bit can
sometimes be set while the visibility map bit is clear after a crash
recovery. The reported values can also disagree because of a change that
occurs after <literal>pg_visibility</literal> examines the visibility map and
before it examines the data page. Any event that causes data corruption
can also cause these bits to disagree.
</para>
<para>
Functions that display information about <literal>PD_ALL_VISIBLE</literal> bits
are much more costly than those that 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(relation 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(relation 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</literal> bit of that block.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><function>pg_visibility_map(relation 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 of the given relation.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><function>pg_visibility(relation 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 of the given relation, plus the <literal>PD_ALL_VISIBLE</literal>
bit of each block.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><function>pg_visibility_map_summary(relation 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(relation regclass, t_ctid OUT tid) returns setof tid</function></term>
<listitem>
<para>
Returns the TIDs of non-frozen tuples stored in pages marked all-frozen
in the visibility map. If this function returns a non-empty set of
TIDs, the visibility map is corrupt.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><function>pg_check_visible(relation regclass, t_ctid OUT tid) returns setof tid</function></term>
<listitem>
<para>
Returns the TIDs of non-all-visible tuples stored in pages marked
all-visible in the visibility map. If this function returns a non-empty
set of TIDs, the visibility map is corrupt.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><function>pg_truncate_visibility_map(relation regclass) returns void</function></term>
<listitem>
<para>
Truncates the visibility map for the given relation. This function is
useful if you believe that the visibility map for the relation is
corrupt and wish to force rebuilding it. The first <command>VACUUM</command>
executed on the given relation after this function is executed will scan
every page in the relation and rebuild the visibility map. (Until that
is done, queries will treat the visibility map as containing all zeroes.)
</para>
</listitem>
</varlistentry>
</variablelist>
<para>
By default, these functions are executable only by superusers and members of the
<literal>pg_stat_scan_tables</literal> role, with the exception of
<function>pg_truncate_visibility_map(relation regclass)</function> which can only
be executed by superusers.
</para>
</sect2>
<sect2>
<title>Author</title>
<para>
Robert Haas <email>rhaas@postgresql.org</email>
</para>
</sect2>
</sect1>
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