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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>F.36. pg_visibility — visibility map information and utilities</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><link rel="prev" href="pgtrgm.html" title="F.35. pg_trgm — support for similarity of text using trigram matching" /><link rel="next" href="pgwalinspect.html" title="F.37. pg_walinspect — low-level WAL inspection" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">F.36. pg_visibility — visibility map information and utilities</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="pgtrgm.html" title="F.35. pg_trgm — support for similarity of text using trigram matching">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="contrib.html" title="Appendix F. Additional Supplied Modules and Extensions">Up</a></td><th width="60%" align="center">Appendix F. Additional Supplied Modules and Extensions</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 16.2 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="pgwalinspect.html" title="F.37. pg_walinspect — low-level WAL inspection">Next</a></td></tr></table><hr /></div><div class="sect1" id="PGVISIBILITY"><div class="titlepage"><div><div><h2 class="title" style="clear: both">F.36. pg_visibility — visibility map information and utilities <a href="#PGVISIBILITY" class="id_link">#</a></h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="sect2"><a href="pgvisibility.html#PGVISIBILITY-FUNCS">F.36.1. Functions</a></span></dt><dt><span class="sect2"><a href="pgvisibility.html#PGVISIBILITY-AUTHOR">F.36.2. Author</a></span></dt></dl></div><a id="id-1.11.7.46.2" class="indexterm"></a><p>
The <code class="filename">pg_visibility</code> 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.
</p><p>
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 <code class="literal">PD_ALL_VISIBLE</code> 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 <code class="literal">pg_visibility</code> examines the visibility map and
before it examines the data page. Any event that causes data corruption
can also cause these bits to disagree.
</p><p>
Functions that display information about <code class="literal">PD_ALL_VISIBLE</code> 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.
</p><div class="sect2" id="PGVISIBILITY-FUNCS"><div class="titlepage"><div><div><h3 class="title">F.36.1. Functions <a href="#PGVISIBILITY-FUNCS" class="id_link">#</a></h3></div></div></div><div class="variablelist"><dl class="variablelist"><dt><span class="term"><code class="function">pg_visibility_map(relation regclass, blkno bigint, all_visible OUT boolean, all_frozen OUT boolean) returns record</code></span></dt><dd><p>
Returns the all-visible and all-frozen bits in the visibility map for
the given block of the given relation.
</p></dd><dt><span class="term"><code class="function">pg_visibility(relation regclass, blkno bigint, all_visible OUT boolean, all_frozen OUT boolean, pd_all_visible OUT boolean) returns record</code></span></dt><dd><p>
Returns the all-visible and all-frozen bits in the visibility map for
the given block of the given relation, plus the
<code class="literal">PD_ALL_VISIBLE</code> bit of that block.
</p></dd><dt><span class="term"><code class="function">pg_visibility_map(relation regclass, blkno OUT bigint, all_visible OUT boolean, all_frozen OUT boolean) returns setof record</code></span></dt><dd><p>
Returns the all-visible and all-frozen bits in the visibility map for
each block of the given relation.
</p></dd><dt><span class="term"><code class="function">pg_visibility(relation regclass, blkno OUT bigint, all_visible OUT boolean, all_frozen OUT boolean, pd_all_visible OUT boolean) returns setof record</code></span></dt><dd><p>
Returns the all-visible and all-frozen bits in the visibility map for
each block of the given relation, plus the <code class="literal">PD_ALL_VISIBLE</code>
bit of each block.
</p></dd><dt><span class="term"><code class="function">pg_visibility_map_summary(relation regclass, all_visible OUT bigint, all_frozen OUT bigint) returns record</code></span></dt><dd><p>
Returns the number of all-visible pages and the number of all-frozen
pages in the relation according to the visibility map.
</p></dd><dt><span class="term"><code class="function">pg_check_frozen(relation regclass, t_ctid OUT tid) returns setof tid</code></span></dt><dd><p>
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.
</p></dd><dt><span class="term"><code class="function">pg_check_visible(relation regclass, t_ctid OUT tid) returns setof tid</code></span></dt><dd><p>
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.
</p></dd><dt><span class="term"><code class="function">pg_truncate_visibility_map(relation regclass) returns void</code></span></dt><dd><p>
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 <code class="command">VACUUM</code>
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.)
</p></dd></dl></div><p>
By default, these functions are executable only by superusers and roles with privileges
of the <code class="literal">pg_stat_scan_tables</code> role, with the exception of
<code class="function">pg_truncate_visibility_map(relation regclass)</code> which can only
be executed by superusers.
</p></div><div class="sect2" id="PGVISIBILITY-AUTHOR"><div class="titlepage"><div><div><h3 class="title">F.36.2. Author <a href="#PGVISIBILITY-AUTHOR" class="id_link">#</a></h3></div></div></div><p>
Robert Haas <code class="email"><<a class="email" href="mailto:rhaas@postgresql.org">rhaas@postgresql.org</a>></code>
</p></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="pgtrgm.html" title="F.35. pg_trgm — support for similarity of text using trigram matching">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="contrib.html" title="Appendix F. Additional Supplied Modules and Extensions">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="pgwalinspect.html" title="F.37. pg_walinspect — low-level WAL inspection">Next</a></td></tr><tr><td width="40%" align="left" valign="top">F.35. pg_trgm —
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