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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>F.10. citext — a case-insensitive character string type</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="btree-gist.html" title="F.9. btree_gist — GiST operator classes with B-tree behavior" /><link rel="next" href="cube.html" title="F.11. cube — a multi-dimensional cube data type" /></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.10. citext — a case-insensitive character string type</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="btree-gist.html" title="F.9. btree_gist — GiST operator classes with B-tree behavior">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 16.3 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="cube.html" title="F.11. cube — a multi-dimensional cube data type">Next</a></td></tr></table><hr /></div><div class="sect1" id="CITEXT"><div class="titlepage"><div><div><h2 class="title" style="clear: both">F.10. citext — a case-insensitive character string type <a href="#CITEXT" class="id_link">#</a></h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="sect2"><a href="citext.html#CITEXT-RATIONALE">F.10.1. Rationale</a></span></dt><dt><span class="sect2"><a href="citext.html#CITEXT-HOW-TO-USE-IT">F.10.2. How to Use It</a></span></dt><dt><span class="sect2"><a href="citext.html#CITEXT-STRING-COMPARISON-BEHAVIOR">F.10.3. String Comparison Behavior</a></span></dt><dt><span class="sect2"><a href="citext.html#CITEXT-LIMITATIONS">F.10.4. Limitations</a></span></dt><dt><span class="sect2"><a href="citext.html#CITEXT-AUTHOR">F.10.5. Author</a></span></dt></dl></div><a id="id-1.11.7.20.2" class="indexterm"></a><p>
The <code class="filename">citext</code> module provides a case-insensitive
character string type, <code class="type">citext</code>. Essentially, it internally calls
<code class="function">lower</code> when comparing values. Otherwise, it behaves almost
exactly like <code class="type">text</code>.
</p><div class="tip"><h3 class="title">Tip</h3><p>
Consider using <em class="firstterm">nondeterministic collations</em> (see
<a class="xref" href="collation.html#COLLATION-NONDETERMINISTIC" title="24.2.2.4. Nondeterministic Collations">Section 24.2.2.4</a>) instead of this module. They
can be used for case-insensitive comparisons, accent-insensitive
comparisons, and other combinations, and they handle more Unicode special
cases correctly.
</p></div><p>
This module is considered <span class="quote">“<span class="quote">trusted</span>”</span>, that is, it can be
installed by non-superusers who have <code class="literal">CREATE</code> privilege
on the current database.
</p><div class="sect2" id="CITEXT-RATIONALE"><div class="titlepage"><div><div><h3 class="title">F.10.1. Rationale <a href="#CITEXT-RATIONALE" class="id_link">#</a></h3></div></div></div><p>
The standard approach to doing case-insensitive matches
in <span class="productname">PostgreSQL</span> has been to use the <code class="function">lower</code>
function when comparing values, for example
</p><pre class="programlisting">
SELECT * FROM tab WHERE lower(col) = LOWER(?);
</pre><p>
</p><p>
This works reasonably well, but has a number of drawbacks:
</p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p>
It makes your SQL statements verbose, and you always have to remember to
use <code class="function">lower</code> on both the column and the query value.
</p></li><li class="listitem"><p>
It won't use an index, unless you create a functional index using
<code class="function">lower</code>.
</p></li><li class="listitem"><p>
If you declare a column as <code class="literal">UNIQUE</code> or <code class="literal">PRIMARY
KEY</code>, the implicitly generated index is case-sensitive. So it's
useless for case-insensitive searches, and it won't enforce
uniqueness case-insensitively.
</p></li></ul></div><p>
The <code class="type">citext</code> data type allows you to eliminate calls
to <code class="function">lower</code> in SQL queries, and allows a primary key to
be case-insensitive. <code class="type">citext</code> is locale-aware, just
like <code class="type">text</code>, which means that the matching of upper case and
lower case characters is dependent on the rules of
the database's <code class="literal">LC_CTYPE</code> setting. Again, this behavior is
identical to the use of <code class="function">lower</code> in queries. But because it's
done transparently by the data type, you don't have to remember to do
anything special in your queries.
</p></div><div class="sect2" id="CITEXT-HOW-TO-USE-IT"><div class="titlepage"><div><div><h3 class="title">F.10.2. How to Use It <a href="#CITEXT-HOW-TO-USE-IT" class="id_link">#</a></h3></div></div></div><p>
Here's a simple example of usage:
</p><pre class="programlisting">
CREATE TABLE users (
nick CITEXT PRIMARY KEY,
pass TEXT NOT NULL
);
INSERT INTO users VALUES ( 'larry', sha256(random()::text::bytea) );
INSERT INTO users VALUES ( 'Tom', sha256(random()::text::bytea) );
INSERT INTO users VALUES ( 'Damian', sha256(random()::text::bytea) );
INSERT INTO users VALUES ( 'NEAL', sha256(random()::text::bytea) );
INSERT INTO users VALUES ( 'Bjørn', sha256(random()::text::bytea) );
SELECT * FROM users WHERE nick = 'Larry';
</pre><p>
The <code class="command">SELECT</code> statement will return one tuple, even though
the <code class="structfield">nick</code> column was set to <code class="literal">larry</code> and the query
was for <code class="literal">Larry</code>.
</p></div><div class="sect2" id="CITEXT-STRING-COMPARISON-BEHAVIOR"><div class="titlepage"><div><div><h3 class="title">F.10.3. String Comparison Behavior <a href="#CITEXT-STRING-COMPARISON-BEHAVIOR" class="id_link">#</a></h3></div></div></div><p>
<code class="type">citext</code> performs comparisons by converting each string to lower
case (as though <code class="function">lower</code> were called) and then comparing the
results normally. Thus, for example, two strings are considered equal
if <code class="function">lower</code> would produce identical results for them.
</p><p>
In order to emulate a case-insensitive collation as closely as possible,
there are <code class="type">citext</code>-specific versions of a number of string-processing
operators and functions. So, for example, the regular expression
operators <code class="literal">~</code> and <code class="literal">~*</code> exhibit the same behavior when
applied to <code class="type">citext</code>: they both match case-insensitively.
The same is true
for <code class="literal">!~</code> and <code class="literal">!~*</code>, as well as for the
<code class="literal">LIKE</code> operators <code class="literal">~~</code> and <code class="literal">~~*</code>, and
<code class="literal">!~~</code> and <code class="literal">!~~*</code>. If you'd like to match
case-sensitively, you can cast the operator's arguments to <code class="type">text</code>.
</p><p>
Similarly, all of the following functions perform matching
case-insensitively if their arguments are <code class="type">citext</code>:
</p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p>
<code class="function">regexp_match()</code>
</p></li><li class="listitem"><p>
<code class="function">regexp_matches()</code>
</p></li><li class="listitem"><p>
<code class="function">regexp_replace()</code>
</p></li><li class="listitem"><p>
<code class="function">regexp_split_to_array()</code>
</p></li><li class="listitem"><p>
<code class="function">regexp_split_to_table()</code>
</p></li><li class="listitem"><p>
<code class="function">replace()</code>
</p></li><li class="listitem"><p>
<code class="function">split_part()</code>
</p></li><li class="listitem"><p>
<code class="function">strpos()</code>
</p></li><li class="listitem"><p>
<code class="function">translate()</code>
</p></li></ul></div><p>
For the regexp functions, if you want to match case-sensitively, you can
specify the <span class="quote">“<span class="quote">c</span>”</span> flag to force a case-sensitive match. Otherwise,
you must cast to <code class="type">text</code> before using one of these functions if
you want case-sensitive behavior.
</p></div><div class="sect2" id="CITEXT-LIMITATIONS"><div class="titlepage"><div><div><h3 class="title">F.10.4. Limitations <a href="#CITEXT-LIMITATIONS" class="id_link">#</a></h3></div></div></div><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p>
<code class="type">citext</code>'s case-folding behavior depends on
the <code class="literal">LC_CTYPE</code> setting of your database. How it compares
values is therefore determined when the database is created.
It is not truly
case-insensitive in the terms defined by the Unicode standard.
Effectively, what this means is that, as long as you're happy with your
collation, you should be happy with <code class="type">citext</code>'s comparisons. But
if you have data in different languages stored in your database, users
of one language may find their query results are not as expected if the
collation is for another language.
</p></li><li class="listitem"><p>
As of <span class="productname">PostgreSQL</span> 9.1, you can attach a
<code class="literal">COLLATE</code> specification to <code class="type">citext</code> columns or data
values. Currently, <code class="type">citext</code> operators will honor a non-default
<code class="literal">COLLATE</code> specification while comparing case-folded strings,
but the initial folding to lower case is always done according to the
database's <code class="literal">LC_CTYPE</code> setting (that is, as though
<code class="literal">COLLATE "default"</code> were given). This may be changed in a
future release so that both steps follow the input <code class="literal">COLLATE</code>
specification.
</p></li><li class="listitem"><p>
<code class="type">citext</code> is not as efficient as <code class="type">text</code> because the
operator functions and the B-tree comparison functions must make copies
of the data and convert it to lower case for comparisons. Also, only
<code class="type">text</code> can support B-Tree deduplication. However,
<code class="type">citext</code> is slightly more efficient than using
<code class="function">lower</code> to get case-insensitive matching.
</p></li><li class="listitem"><p>
<code class="type">citext</code> doesn't help much if you need data to compare
case-sensitively in some contexts and case-insensitively in other
contexts. The standard answer is to use the <code class="type">text</code> type and
manually use the <code class="function">lower</code> function when you need to compare
case-insensitively; this works all right if case-insensitive comparison
is needed only infrequently. If you need case-insensitive behavior most
of the time and case-sensitive infrequently, consider storing the data
as <code class="type">citext</code> and explicitly casting the column to <code class="type">text</code>
when you want case-sensitive comparison. In either situation, you will
need two indexes if you want both types of searches to be fast.
</p></li><li class="listitem"><p>
The schema containing the <code class="type">citext</code> operators must be
in the current <code class="varname">search_path</code> (typically <code class="literal">public</code>);
if it is not, the normal case-sensitive <code class="type">text</code> operators
will be invoked instead.
</p></li><li class="listitem"><p>
The approach of lower-casing strings for comparison does not handle some
Unicode special cases correctly, for example when one upper-case letter
has two lower-case letter equivalents. Unicode distinguishes between
<em class="firstterm">case mapping</em> and <em class="firstterm">case
folding</em> for this reason. Use nondeterministic collations
instead of <code class="type">citext</code> to handle that correctly.
</p></li></ul></div></div><div class="sect2" id="CITEXT-AUTHOR"><div class="titlepage"><div><div><h3 class="title">F.10.5. Author <a href="#CITEXT-AUTHOR" class="id_link">#</a></h3></div></div></div><p>
David E. Wheeler <code class="email"><<a class="email" href="mailto:david@kineticode.com">david@kineticode.com</a>></code>
</p><p>
Inspired by the original <code class="type">citext</code> module by Donald Fraser.
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