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Green_GoodERP18_FSJ_Standard/PostgreSQL/doc/postgresql/html/pgvisibility.html

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  1. <?xml version="1.0" encoding="UTF-8" standalone="no"?>

  2. <!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.32. pg_visibility</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 V1.79.1" /><link rel="prev" href="pgtrgm.html" title="F.31. pg_trgm" /><link rel="next" href="postgres-fdw.html" title="F.33. postgres_fdw" /></head><body><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">F.32. pg_visibility</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="pgtrgm.html" title="F.31. pg_trgm">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="contrib.html" title="Appendix F. Additional Supplied Modules">Up</a></td><th width="60%" align="center">Appendix F. Additional Supplied Modules</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 12.4 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="postgres-fdw.html" title="F.33. postgres_fdw">Next</a></td></tr></table><hr></hr></div><div class="sect1" id="PGVISIBILITY"><div class="titlepage"><div><div><h2 class="title" style="clear: both">F.32. pg_visibility</h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="sect2"><a href="pgvisibility.html#id-1.11.7.41.6">F.32.1. Functions</a></span></dt><dt><span class="sect2"><a href="pgvisibility.html#id-1.11.7.41.7">F.32.2. Author</a></span></dt></dl></div><a id="id-1.11.7.41.2" class="indexterm"></a><p>

  3.   The <code class="filename">pg_visibility</code> module provides a means for examining the

  4.   visibility map (VM) and page-level visibility information of a table.

  5.   It also provides functions to check the integrity of a visibility map and to

  6.   force it to be rebuilt.

  7.  </p><p>

  8.   Three different bits are used to store information about page-level

  9.   visibility.  The all-visible bit in the visibility map indicates that every

  10.   tuple in the corresponding page of the relation is visible to every current

  11.   and future transaction.  The all-frozen bit in the visibility map indicates

  12.   that every tuple in the page is frozen; that is, no future vacuum will need

  13.   to modify the page until such time as a tuple is inserted, updated, deleted,

  14.   or locked on that page.

  15.   The page header's <code class="literal">PD_ALL_VISIBLE</code> bit has the

  16.   same meaning as the all-visible bit in the visibility map, but is stored

  17.   within the data page itself rather than in a separate data structure.

  18.   These two bits will normally agree, but the page's all-visible bit can

  19.   sometimes be set while the visibility map bit is clear after a crash

  20.   recovery.  The reported values can also disagree because of a change that

  21.   occurs after <code class="literal">pg_visibility</code> examines the visibility map and

  22.   before it examines the data page.  Any event that causes data corruption

  23.   can also cause these bits to disagree.

  24.  </p><p>

  25.   Functions that display information about <code class="literal">PD_ALL_VISIBLE</code> bits

  26.   are much more costly than those that only consult the visibility map,

  27.   because they must read the relation's data blocks rather than only the

  28.   (much smaller) visibility map.  Functions that check the relation's

  29.   data blocks are similarly expensive.

  30.  </p><div class="sect2" id="id-1.11.7.41.6"><div class="titlepage"><div><div><h3 class="title">F.32.1. Functions</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>

  31.       Returns the all-visible and all-frozen bits in the visibility map for

  32.       the given block of the given relation.

  33.      </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>

  34.       Returns the all-visible and all-frozen bits in the visibility map for

  35.       the given block of the given relation, plus the

  36.       <code class="literal">PD_ALL_VISIBLE</code> bit of that block.

  37.      </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>

  38.       Returns the all-visible and all-frozen bits in the visibility map for

  39.       each block of the given relation.

  40.      </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>

  41.       Returns the all-visible and all-frozen bits in the visibility map for

  42.       each block of the given relation, plus the <code class="literal">PD_ALL_VISIBLE</code>

  43.       bit of each block.

  44.      </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>

  45.       Returns the number of all-visible pages and the number of all-frozen

  46.       pages in the relation according to the visibility map.

  47.      </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>

  48.       Returns the TIDs of non-frozen tuples stored in pages marked all-frozen

  49.       in the visibility map.  If this function returns a non-empty set of

  50.       TIDs, the visibility map is corrupt.

  51.      </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>

  52.       Returns the TIDs of non-all-visible tuples stored in pages marked

  53.       all-visible in the visibility map.  If this function returns a non-empty

  54.       set of TIDs, the visibility map is corrupt.

  55.      </p></dd><dt><span class="term"><code class="function">pg_truncate_visibility_map(relation regclass) returns void</code></span></dt><dd><p>

  56.       Truncates the visibility map for the given relation.  This function is

  57.       useful if you believe that the visibility map for the relation is

  58.       corrupt and wish to force rebuilding it.  The first <code class="command">VACUUM</code>

  59.       executed on the given relation after this function is executed will scan

  60.       every page in the relation and rebuild the visibility map.  (Until that

  61.       is done, queries will treat the visibility map as containing all zeroes.)

  62.      </p></dd></dl></div><p>

  63.    By default, these functions are executable only by superusers and members of the

  64.    <code class="literal">pg_stat_scan_tables</code> role, with the exception of

  65.    <code class="function">pg_truncate_visibility_map(relation regclass)</code> which can only

  66.    be executed by superusers.

  67.   </p></div><div class="sect2" id="id-1.11.7.41.7"><div class="titlepage"><div><div><h3 class="title">F.32.2. Author</h3></div></div></div><p>

  68.    Robert Haas <code class="email">&lt;<a class="email" href="mailto:rhaas@postgresql.org">rhaas@postgresql.org</a>&gt;</code>

  69.   </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">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="contrib.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="postgres-fdw.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">F.31. pg_trgm </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> F.33. postgres_fdw</td></tr></table></div></body></html>
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