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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>73.7. Heap-Only Tuples (HOT)</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="storage-page-layout.html" title="73.6. Database Page Layout" /><link rel="next" href="bki.html" title="Chapter 74. System Catalog Declarations and Initial Contents" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">73.7. Heap-Only Tuples (<acronym class="acronym">HOT</acronym>)</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="storage-page-layout.html" title="73.6. Database Page Layout">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="storage.html" title="Chapter 73. Database Physical Storage">Up</a></td><th width="60%" align="center">Chapter 73. Database Physical Storage</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 15.5 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="bki.html" title="Chapter 74. System Catalog Declarations and Initial Contents">Next</a></td></tr></table><hr /></div><div class="sect1" id="STORAGE-HOT"><div class="titlepage"><div><div><h2 class="title" style="clear: both">73.7. Heap-Only Tuples (<acronym class="acronym">HOT</acronym>)</h2></div></div></div><p>
+  To allow for high concurrency, <span class="productname">PostgreSQL</span>
+  uses <a class="link" href="mvcc-intro.html" title="13.1. Introduction">multiversion concurrency
+  control</a> (<acronym class="acronym">MVCC</acronym>) to store rows.  However,
+  <acronym class="acronym">MVCC</acronym> has some downsides for update queries.
+  Specifically, updates require new versions of rows to be added to
+  tables.  This can also require new index entries for each updated row,
+  and removal of old versions of rows and their index entries can be
+  expensive.
+ </p><p>
+  To help reduce the overhead of updates,
+  <span class="productname">PostgreSQL</span> has an optimization called
+  heap-only tuples (<acronym class="acronym">HOT</acronym>).  This optimization is
+  possible when:
+
+  </p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p>
+     The update does not modify any columns referenced by the table's
+     indexes, including expression and partial indexes.
+     </p></li><li class="listitem"><p>
+     There is sufficient free space on the page containing the old row
+     for the updated row.
+    </p></li></ul></div><p>
+
+  In such cases, heap-only tuples provide two optimizations:
+
+  </p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p>
+     New index entries are not needed to represent updated rows.
+    </p></li><li class="listitem"><p>
+     Old versions of updated rows can be completely removed during normal
+     operation, including <code class="command">SELECT</code>s, instead of requiring
+     periodic vacuum operations.  (This is possible because indexes
+     do not reference their <a class="link" href="storage-page-layout.html" title="73.6. Database Page Layout">page
+     item identifiers</a>.)
+    </p></li></ul></div><p>
+ </p><p>
+  In summary, heap-only tuple updates can only be created
+  if columns used by indexes are not updated.  You can
+  increase the likelihood of sufficient page space for
+  <acronym class="acronym">HOT</acronym> updates by decreasing a table's <a class="link" href="sql-createtable.html#RELOPTION-FILLFACTOR"><code class="literal">fillfactor</code></a>.
+  If you don't, <acronym class="acronym">HOT</acronym> updates will still happen because
+  new rows will naturally migrate to new pages and existing pages with
+  sufficient free space for new row versions.  The system view <a class="link" href="monitoring-stats.html#MONITORING-PG-STAT-ALL-TABLES-VIEW" title="28.2.17. pg_stat_all_tables">pg_stat_all_tables</a>
+  allows monitoring of the occurrence of HOT and non-HOT updates.
+ </p></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="storage-page-layout.html" title="73.6. Database Page Layout">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="storage.html" title="Chapter 73. Database Physical Storage">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="bki.html" title="Chapter 74. System Catalog Declarations and Initial Contents">Next</a></td></tr><tr><td width="40%" align="left" valign="top">73.6. Database Page Layout </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 15.5 Documentation">Home</a></td><td width="40%" align="right" valign="top"> Chapter 74. System Catalog Declarations and Initial Contents</td></tr></table></div></body></html>
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