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@@ -1,5 +1,5 @@
<?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>5.4. Constraints</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="ddl-generated-columns.html" title="5.3. Generated Columns" /><link rel="next" href="ddl-system-columns.html" title="5.5. System Columns" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">5.4. Constraints</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="ddl-generated-columns.html" title="5.3. Generated Columns">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="ddl.html" title="Chapter 5. Data Definition">Up</a></td><th width="60%" align="center">Chapter 5. Data Definition</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="ddl-system-columns.html" title="5.5. System Columns">Next</a></td></tr></table><hr /></div><div class="sect1" id="DDL-CONSTRAINTS"><div class="titlepage"><div><div><h2 class="title" style="clear: both">5.4. Constraints</h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="sect2"><a href="ddl-constraints.html#DDL-CONSTRAINTS-CHECK-CONSTRAINTS">5.4.1. Check Constraints</a></span></dt><dt><span class="sect2"><a href="ddl-constraints.html#id-1.5.4.6.6">5.4.2. Not-Null Constraints</a></span></dt><dt><span class="sect2"><a href="ddl-constraints.html#DDL-CONSTRAINTS-UNIQUE-CONSTRAINTS">5.4.3. Unique Constraints</a></span></dt><dt><span class="sect2"><a href="ddl-constraints.html#DDL-CONSTRAINTS-PRIMARY-KEYS">5.4.4. Primary Keys</a></span></dt><dt><span class="sect2"><a href="ddl-constraints.html#DDL-CONSTRAINTS-FK">5.4.5. Foreign Keys</a></span></dt><dt><span class="sect2"><a href="ddl-constraints.html#DDL-CONSTRAINTS-EXCLUSION">5.4.6. Exclusion Constraints</a></span></dt></dl></div><a id="id-1.5.4.6.2" class="indexterm"></a><p>
+<!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>5.4. Constraints</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="ddl-generated-columns.html" title="5.3. Generated Columns" /><link rel="next" href="ddl-system-columns.html" title="5.5. System Columns" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">5.4. Constraints</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="ddl-generated-columns.html" title="5.3. Generated Columns">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="ddl.html" title="Chapter 5. Data Definition">Up</a></td><th width="60%" align="center">Chapter 5. Data Definition</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 15.6 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="ddl-system-columns.html" title="5.5. System Columns">Next</a></td></tr></table><hr /></div><div class="sect1" id="DDL-CONSTRAINTS"><div class="titlepage"><div><div><h2 class="title" style="clear: both">5.4. Constraints</h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="sect2"><a href="ddl-constraints.html#DDL-CONSTRAINTS-CHECK-CONSTRAINTS">5.4.1. Check Constraints</a></span></dt><dt><span class="sect2"><a href="ddl-constraints.html#id-1.5.4.6.6">5.4.2. Not-Null Constraints</a></span></dt><dt><span class="sect2"><a href="ddl-constraints.html#DDL-CONSTRAINTS-UNIQUE-CONSTRAINTS">5.4.3. Unique Constraints</a></span></dt><dt><span class="sect2"><a href="ddl-constraints.html#DDL-CONSTRAINTS-PRIMARY-KEYS">5.4.4. Primary Keys</a></span></dt><dt><span class="sect2"><a href="ddl-constraints.html#DDL-CONSTRAINTS-FK">5.4.5. Foreign Keys</a></span></dt><dt><span class="sect2"><a href="ddl-constraints.html#DDL-CONSTRAINTS-EXCLUSION">5.4.6. Exclusion Constraints</a></span></dt></dl></div><a id="id-1.5.4.6.2" class="indexterm"></a><p>
Data types are a way to limit the kind of data that can be stored
in a table. For many applications, however, the constraint they
provide is too coarse. For example, a column containing a product
@@ -573,16 +573,16 @@ CREATE TABLE posts (
referencing column(s) as <code class="literal">NOT NULL</code>.
</p><p>
A foreign key must reference columns that either are a primary key or
- form a unique constraint. This means that the referenced columns always
- have an index (the one underlying the primary key or unique constraint);
- so checks on whether a referencing row has a match will be efficient.
- Since a <code class="command">DELETE</code> of a row from the referenced table
- or an <code class="command">UPDATE</code> of a referenced column will require
- a scan of the referencing table for rows matching the old value, it
- is often a good idea to index the referencing columns too. Because this
- is not always needed, and there are many choices available on how
- to index, declaration of a foreign key constraint does not
- automatically create an index on the referencing columns.
+ form a unique constraint, or are columns from a non-partial unique index.
+ This means that the referenced columns always have an index to allow
+ efficient lookups on whether a referencing row has a match. Since a
+ <code class="command">DELETE</code> of a row from the referenced table or an
+ <code class="command">UPDATE</code> of a referenced column will require a scan of
+ the referencing table for rows matching the old value, it is often a good
+ idea to index the referencing columns too. Because this is not always
+ needed, and there are many choices available on how to index, the
+ declaration of a foreign key constraint does not automatically create an
+ index on the referencing columns.
</p><p>
More information about updating and deleting data is in <a class="xref" href="dml.html" title="Chapter 6. Data Manipulation">Chapter 6</a>. Also see the description of foreign key constraint
syntax in the reference documentation for
@@ -604,4 +604,4 @@ CREATE TABLE circles (
</p><p>
Adding an exclusion constraint will automatically create an index
of the type specified in the constraint declaration.
- </p></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="ddl-generated-columns.html" title="5.3. Generated Columns">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="ddl.html" title="Chapter 5. Data Definition">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="ddl-system-columns.html" title="5.5. System Columns">Next</a></td></tr><tr><td width="40%" align="left" valign="top">5.3. Generated Columns </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"> 5.5. System Columns</td></tr></table></div></body></html> \ No newline at end of file
+ </p></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="ddl-generated-columns.html" title="5.3. Generated Columns">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="ddl.html" title="Chapter 5. Data Definition">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="ddl-system-columns.html" title="5.5. System Columns">Next</a></td></tr><tr><td width="40%" align="left" valign="top">5.3. Generated Columns </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 15.6 Documentation">Home</a></td><td width="40%" align="right" valign="top"> 5.5. System Columns</td></tr></table></div></body></html> \ No newline at end of file