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| author | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-04-13 13:44:03 +0000 |
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| committer | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-04-13 13:44:03 +0000 |
| commit | 293913568e6a7a86fd1479e1cff8e2ecb58d6568 (patch) | |
| tree | fc3b469a3ec5ab71b36ea97cc7aaddb838423a0c /doc/src/sgml/html/pgvisibility.html | |
| parent | Initial commit. (diff) | |
| download | postgresql-16-293913568e6a7a86fd1479e1cff8e2ecb58d6568.tar.xz postgresql-16-293913568e6a7a86fd1479e1cff8e2ecb58d6568.zip | |
Adding upstream version 16.2.upstream/16.2
Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
Diffstat (limited to 'doc/src/sgml/html/pgvisibility.html')
| -rw-r--r-- | doc/src/sgml/html/pgvisibility.html | 70 |
1 files changed, 70 insertions, 0 deletions
diff --git a/doc/src/sgml/html/pgvisibility.html b/doc/src/sgml/html/pgvisibility.html new file mode 100644 index 0000000..32aebf4 --- /dev/null +++ b/doc/src/sgml/html/pgvisibility.html @@ -0,0 +1,70 @@ +<?xml version="1.0" encoding="UTF-8" standalone="no"?> +<!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 — + support for similarity of text using trigram matching </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 16.2 Documentation">Home</a></td><td width="40%" align="right" valign="top"> F.37. pg_walinspect — low-level WAL inspection</td></tr></table></div></body></html>
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