diff options
| author | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-05-04 12:15:05 +0000 |
|---|---|---|
| committer | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-05-04 12:15:05 +0000 |
| commit | 46651ce6fe013220ed397add242004d764fc0153 (patch) | |
| tree | 6e5299f990f88e60174a1d3ae6e48eedd2688b2b /doc/src/sgml/html/indexes-expressional.html | |
| parent | Initial commit. (diff) | |
| download | postgresql-14-upstream.tar.xz postgresql-14-upstream.zip | |
Adding upstream version 14.5.upstream/14.5upstream
Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
Diffstat (limited to 'doc/src/sgml/html/indexes-expressional.html')
| -rw-r--r-- | doc/src/sgml/html/indexes-expressional.html | 49 |
1 files changed, 49 insertions, 0 deletions
diff --git a/doc/src/sgml/html/indexes-expressional.html b/doc/src/sgml/html/indexes-expressional.html new file mode 100644 index 0000000..2fb32d5 --- /dev/null +++ b/doc/src/sgml/html/indexes-expressional.html @@ -0,0 +1,49 @@ +<?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>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 xmlns="http://www.w3.org/TR/xhtml1/transitional" 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 14.5 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></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 non-HOT update. 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 xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navfooter"><hr></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 14.5 Documentation">Home</a></td><td width="40%" align="right" valign="top"> 11.8. Partial Indexes</td></tr></table></div></body></html>
\ No newline at end of file |