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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>11.7. Indexes on Expressions</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="indexes-unique.html" title="11.6. Unique Indexes" /><link rel="next" href="indexes-partial.html" title="11.8. Partial Indexes" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">11.7. Indexes on Expressions</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="indexes-unique.html" title="11.6. Unique Indexes">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="indexes.html" title="Chapter 11. Indexes">Up</a></td><th width="60%" align="center">Chapter 11. Indexes</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="indexes-partial.html" title="11.8. Partial Indexes">Next</a></td></tr></table><hr /></div><div class="sect1" id="INDEXES-EXPRESSIONAL"><div class="titlepage"><div><div><h2 class="title" style="clear: both">11.7. Indexes on Expressions</h2></div></div></div><a id="id-1.5.10.10.2" class="indexterm"></a><p>
+   An index column need not be just a column of the underlying table,
+   but can be a function or scalar expression computed from one or
+   more columns of the table.  This feature is useful to obtain fast
+   access to tables based on the results of computations.
+  </p><p>
+   For example, a common way to do case-insensitive comparisons is to
+   use the <code class="function">lower</code> function:
+</p><pre class="programlisting">
+SELECT * FROM test1 WHERE lower(col1) = 'value';
+</pre><p>
+   This query can use an index if one has been
+   defined on the result of the <code class="literal">lower(col1)</code>
+   function:
+</p><pre class="programlisting">
+CREATE INDEX test1_lower_col1_idx ON test1 (lower(col1));
+</pre><p>
+  </p><p>
+   If we were to declare this index <code class="literal">UNIQUE</code>, it would prevent
+   creation of rows whose <code class="literal">col1</code> values differ only in case,
+   as well as rows whose <code class="literal">col1</code> values are actually identical.
+   Thus, indexes on expressions can be used to enforce constraints that
+   are not definable as simple unique constraints.
+  </p><p>
+   As another example, if one often does queries like:
+</p><pre class="programlisting">
+SELECT * FROM people WHERE (first_name || ' ' || last_name) = 'John Smith';
+</pre><p>
+   then it might be worth creating an index like this:
+</p><pre class="programlisting">
+CREATE INDEX people_names ON people ((first_name || ' ' || last_name));
+</pre><p>
+  </p><p>
+   The syntax of the <code class="command">CREATE INDEX</code> command normally requires
+   writing parentheses around index expressions, as shown in the second
+   example.  The parentheses can be omitted when the expression is just
+   a function call, as in the first example.
+  </p><p>
+   Index expressions are relatively expensive to maintain, because the
+   derived expression(s) must be computed for each row insertion
+   and <a class="link" href="storage-hot.html" title="73.7. Heap-Only Tuples (HOT)">non-HOT update.</a>  However, the index expressions are
+   <span class="emphasis"><em>not</em></span> recomputed during an indexed search, since they are
+   already stored in the index.  In both examples above, the system
+   sees the query as just <code class="literal">WHERE indexedcolumn = 'constant'</code>
+   and so the speed of the search is equivalent to any other simple index
+   query.  Thus, indexes on expressions are useful when retrieval speed
+   is more important than insertion and update speed.
+  </p></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="indexes-unique.html" title="11.6. Unique Indexes">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="indexes.html" title="Chapter 11. Indexes">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="indexes-partial.html" title="11.8. Partial Indexes">Next</a></td></tr><tr><td width="40%" align="left" valign="top">11.6. Unique Indexes </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"> 11.8. Partial Indexes</td></tr></table></div></body></html>
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