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<?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>24.2. Collation Support</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="locale.html" title="24.1. Locale Support" /><link rel="next" href="multibyte.html" title="24.3. Character Set Support" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">24.2. Collation Support</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="locale.html" title="24.1. Locale Support">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="charset.html" title="Chapter 24. Localization">Up</a></td><th width="60%" align="center">Chapter 24. Localization</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 15.7 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="multibyte.html" title="24.3. Character Set Support">Next</a></td></tr></table><hr /></div><div class="sect1" id="COLLATION"><div class="titlepage"><div><div><h2 class="title" style="clear: both">24.2. Collation Support</h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="sect2"><a href="collation.html#id-1.6.11.4.4">24.2.1. Concepts</a></span></dt><dt><span class="sect2"><a href="collation.html#COLLATION-MANAGING">24.2.2. Managing Collations</a></span></dt></dl></div><a id="id-1.6.11.4.2" class="indexterm"></a><p>
The collation feature allows specifying the sort order and character
classification behavior of data per-column, or even per-operation.
This alleviates the restriction that the
<code class="symbol">LC_COLLATE</code> and <code class="symbol">LC_CTYPE</code> settings
of a database cannot be changed after its creation.
</p><div class="sect2" id="id-1.6.11.4.4"><div class="titlepage"><div><div><h3 class="title">24.2.1. Concepts</h3></div></div></div><p>
Conceptually, every expression of a collatable data type has a
collation. (The built-in collatable data types are
<code class="type">text</code>, <code class="type">varchar</code>, and <code class="type">char</code>.
User-defined base types can also be marked collatable, and of course
a <a class="glossterm" href="glossary.html#GLOSSARY-DOMAIN"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-DOMAIN" title="Domain">domain</a></em></a> over a
collatable data type is collatable.) If the
expression is a column reference, the collation of the expression is the
defined collation of the column. If the expression is a constant, the
collation is the default collation of the data type of the
constant. The collation of a more complex expression is derived
from the collations of its inputs, as described below.
</p><p>
The collation of an expression can be the <span class="quote">“<span class="quote">default</span>”</span>
collation, which means the locale settings defined for the
database. It is also possible for an expression's collation to be
indeterminate. In such cases, ordering operations and other
operations that need to know the collation will fail.
</p><p>
When the database system has to perform an ordering or a character
classification, it uses the collation of the input expression. This
happens, for example, with <code class="literal">ORDER BY</code> clauses
and function or operator calls such as <code class="literal"><</code>.
The collation to apply for an <code class="literal">ORDER BY</code> clause
is simply the collation of the sort key. The collation to apply for a
function or operator call is derived from the arguments, as described
below. In addition to comparison operators, collations are taken into
account by functions that convert between lower and upper case
letters, such as <code class="function">lower</code>, <code class="function">upper</code>, and
<code class="function">initcap</code>; by pattern matching operators; and by
<code class="function">to_char</code> and related functions.
</p><p>
For a function or operator call, the collation that is derived by
examining the argument collations is used at run time for performing
the specified operation. If the result of the function or operator
call is of a collatable data type, the collation is also used at parse
time as the defined collation of the function or operator expression,
in case there is a surrounding expression that requires knowledge of
its collation.
</p><p>
The <em class="firstterm">collation derivation</em> of an expression can be
implicit or explicit. This distinction affects how collations are
combined when multiple different collations appear in an
expression. An explicit collation derivation occurs when a
<code class="literal">COLLATE</code> clause is used; all other collation
derivations are implicit. When multiple collations need to be
combined, for example in a function call, the following rules are
used:
</p><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem"><p>
If any input expression has an explicit collation derivation, then
all explicitly derived collations among the input expressions must be
the same, otherwise an error is raised. If any explicitly
derived collation is present, that is the result of the
collation combination.
</p></li><li class="listitem"><p>
Otherwise, all input expressions must have the same implicit
collation derivation or the default collation. If any non-default
collation is present, that is the result of the collation combination.
Otherwise, the result is the default collation.
</p></li><li class="listitem"><p>
If there are conflicting non-default implicit collations among the
input expressions, then the combination is deemed to have indeterminate
collation. This is not an error condition unless the particular
function being invoked requires knowledge of the collation it should
apply. If it does, an error will be raised at run-time.
</p></li></ol></div><p>
For example, consider this table definition:
</p><pre class="programlisting">
CREATE TABLE test1 (
a text COLLATE "de_DE",
b text COLLATE "es_ES",
...
);
</pre><p>
Then in
</p><pre class="programlisting">
SELECT a < 'foo' FROM test1;
</pre><p>
the <code class="literal"><</code> comparison is performed according to
<code class="literal">de_DE</code> rules, because the expression combines an
implicitly derived collation with the default collation. But in
</p><pre class="programlisting">
SELECT a < ('foo' COLLATE "fr_FR") FROM test1;
</pre><p>
the comparison is performed using <code class="literal">fr_FR</code> rules,
because the explicit collation derivation overrides the implicit one.
Furthermore, given
</p><pre class="programlisting">
SELECT a < b FROM test1;
</pre><p>
the parser cannot determine which collation to apply, since the
<code class="structfield">a</code> and <code class="structfield">b</code> columns have conflicting
implicit collations. Since the <code class="literal"><</code> operator
does need to know which collation to use, this will result in an
error. The error can be resolved by attaching an explicit collation
specifier to either input expression, thus:
</p><pre class="programlisting">
SELECT a < b COLLATE "de_DE" FROM test1;
</pre><p>
or equivalently
</p><pre class="programlisting">
SELECT a COLLATE "de_DE" < b FROM test1;
</pre><p>
On the other hand, the structurally similar case
</p><pre class="programlisting">
SELECT a || b FROM test1;
</pre><p>
does not result in an error, because the <code class="literal">||</code> operator
does not care about collations: its result is the same regardless
of the collation.
</p><p>
The collation assigned to a function or operator's combined input
expressions is also considered to apply to the function or operator's
result, if the function or operator delivers a result of a collatable
data type. So, in
</p><pre class="programlisting">
SELECT * FROM test1 ORDER BY a || 'foo';
</pre><p>
the ordering will be done according to <code class="literal">de_DE</code> rules.
But this query:
</p><pre class="programlisting">
SELECT * FROM test1 ORDER BY a || b;
</pre><p>
results in an error, because even though the <code class="literal">||</code> operator
doesn't need to know a collation, the <code class="literal">ORDER BY</code> clause does.
As before, the conflict can be resolved with an explicit collation
specifier:
</p><pre class="programlisting">
SELECT * FROM test1 ORDER BY a || b COLLATE "fr_FR";
</pre><p>
</p></div><div class="sect2" id="COLLATION-MANAGING"><div class="titlepage"><div><div><h3 class="title">24.2.2. Managing Collations</h3></div></div></div><p>
A collation is an SQL schema object that maps an SQL name to locales
provided by libraries installed in the operating system. A collation
definition has a <em class="firstterm">provider</em> that specifies which
library supplies the locale data. One standard provider name
is <code class="literal">libc</code>, which uses the locales provided by the
operating system C library. These are the locales used by most tools
provided by the operating system. Another provider
is <code class="literal">icu</code>, which uses the external
ICU<a id="id-1.6.11.4.5.2.4" class="indexterm"></a> library. ICU locales can only be
used if support for ICU was configured when PostgreSQL was built.
</p><p>
A collation object provided by <code class="literal">libc</code> maps to a
combination of <code class="symbol">LC_COLLATE</code> and <code class="symbol">LC_CTYPE</code>
settings, as accepted by the <code class="literal">setlocale()</code> system library call. (As
the name would suggest, the main purpose of a collation is to set
<code class="symbol">LC_COLLATE</code>, which controls the sort order. But
it is rarely necessary in practice to have an
<code class="symbol">LC_CTYPE</code> setting that is different from
<code class="symbol">LC_COLLATE</code>, so it is more convenient to collect
these under one concept than to create another infrastructure for
setting <code class="symbol">LC_CTYPE</code> per expression.) Also,
a <code class="literal">libc</code> collation
is tied to a character set encoding (see <a class="xref" href="multibyte.html" title="24.3. Character Set Support">Section 24.3</a>).
The same collation name may exist for different encodings.
</p><p>
A collation object provided by <code class="literal">icu</code> maps to a named
collator provided by the ICU library. ICU does not support
separate <span class="quote">“<span class="quote">collate</span>”</span> and <span class="quote">“<span class="quote">ctype</span>”</span> settings, so
they are always the same. Also, ICU collations are independent of the
encoding, so there is always only one ICU collation of a given name in
a database.
</p><div class="sect3" id="id-1.6.11.4.5.5"><div class="titlepage"><div><div><h4 class="title">24.2.2.1. Standard Collations</h4></div></div></div><p>
On all platforms, the collations named <code class="literal">default</code>,
<code class="literal">C</code>, and <code class="literal">POSIX</code> are available. Additional
collations may be available depending on operating system support.
The <code class="literal">default</code> collation selects the <code class="symbol">LC_COLLATE</code>
and <code class="symbol">LC_CTYPE</code> values specified at database creation time.
The <code class="literal">C</code> and <code class="literal">POSIX</code> collations both specify
<span class="quote">“<span class="quote">traditional C</span>”</span> behavior, in which only the ASCII letters
<span class="quote">“<span class="quote"><code class="literal">A</code></span>”</span> through <span class="quote">“<span class="quote"><code class="literal">Z</code></span>”</span>
are treated as letters, and sorting is done strictly by character
code byte values.
</p><p>
Additionally, the SQL standard collation name <code class="literal">ucs_basic</code>
is available for encoding <code class="literal">UTF8</code>. It is equivalent
to <code class="literal">C</code> and sorts by Unicode code point.
</p></div><div class="sect3" id="id-1.6.11.4.5.6"><div class="titlepage"><div><div><h4 class="title">24.2.2.2. Predefined Collations</h4></div></div></div><p>
If the operating system provides support for using multiple locales
within a single program (<code class="function">newlocale</code> and related functions),
or if support for ICU is configured,
then when a database cluster is initialized, <code class="command">initdb</code>
populates the system catalog <code class="literal">pg_collation</code> with
collations based on all the locales it finds in the operating
system at the time.
</p><p>
To inspect the currently available locales, use the query <code class="literal">SELECT
* FROM pg_collation</code>, or the command <code class="command">\dOS+</code>
in <span class="application">psql</span>.
</p><div class="sect4" id="id-1.6.11.4.5.6.4"><div class="titlepage"><div><div><h5 class="title">24.2.2.2.1. libc Collations</h5></div></div></div><p>
For example, the operating system might
provide a locale named <code class="literal">de_DE.utf8</code>.
<code class="command">initdb</code> would then create a collation named
<code class="literal">de_DE.utf8</code> for encoding <code class="literal">UTF8</code>
that has both <code class="symbol">LC_COLLATE</code> and
<code class="symbol">LC_CTYPE</code> set to <code class="literal">de_DE.utf8</code>.
It will also create a collation with the <code class="literal">.utf8</code>
tag stripped off the name. So you could also use the collation
under the name <code class="literal">de_DE</code>, which is less cumbersome
to write and makes the name less encoding-dependent. Note that,
nevertheless, the initial set of collation names is
platform-dependent.
</p><p>
The default set of collations provided by <code class="literal">libc</code> map
directly to the locales installed in the operating system, which can be
listed using the command <code class="literal">locale -a</code>. In case
a <code class="literal">libc</code> collation is needed that has different values
for <code class="symbol">LC_COLLATE</code> and <code class="symbol">LC_CTYPE</code>, or if new
locales are installed in the operating system after the database system
was initialized, then a new collation may be created using
the <a class="xref" href="sql-createcollation.html" title="CREATE COLLATION"><span class="refentrytitle">CREATE COLLATION</span></a> command.
New operating system locales can also be imported en masse using
the <a class="link" href="functions-admin.html#FUNCTIONS-ADMIN-COLLATION" title="Table 9.96. Collation Management Functions"><code class="function">pg_import_system_collations()</code></a> function.
</p><p>
Within any particular database, only collations that use that
database's encoding are of interest. Other entries in
<code class="literal">pg_collation</code> are ignored. Thus, a stripped collation
name such as <code class="literal">de_DE</code> can be considered unique
within a given database even though it would not be unique globally.
Use of the stripped collation names is recommended, since it will
make one fewer thing you need to change if you decide to change to
another database encoding. Note however that the <code class="literal">default</code>,
<code class="literal">C</code>, and <code class="literal">POSIX</code> collations can be used regardless of
the database encoding.
</p><p>
<span class="productname">PostgreSQL</span> considers distinct collation
objects to be incompatible even when they have identical properties.
Thus for example,
</p><pre class="programlisting">
SELECT a COLLATE "C" < b COLLATE "POSIX" FROM test1;
</pre><p>
will draw an error even though the <code class="literal">C</code> and <code class="literal">POSIX</code>
collations have identical behaviors. Mixing stripped and non-stripped
collation names is therefore not recommended.
</p></div><div class="sect4" id="id-1.6.11.4.5.6.5"><div class="titlepage"><div><div><h5 class="title">24.2.2.2.2. ICU Collations</h5></div></div></div><p>
With ICU, it is not sensible to enumerate all possible locale names. ICU
uses a particular naming system for locales, but there are many more ways
to name a locale than there are actually distinct locales.
<code class="command">initdb</code> uses the ICU APIs to extract a set of distinct
locales to populate the initial set of collations. Collations provided by
ICU are created in the SQL environment with names in BCP 47 language tag
format, with a <span class="quote">“<span class="quote">private use</span>”</span>
extension <code class="literal">-x-icu</code> appended, to distinguish them from
libc locales.
</p><p>
Here are some example collations that might be created:
</p><div class="variablelist"><dl class="variablelist"><dt><span class="term"><code class="literal">de-x-icu</code></span></dt><dd><p>German collation, default variant</p></dd><dt><span class="term"><code class="literal">de-AT-x-icu</code></span></dt><dd><p>German collation for Austria, default variant</p><p>
(There are also, say, <code class="literal">de-DE-x-icu</code>
or <code class="literal">de-CH-x-icu</code>, but as of this writing, they are
equivalent to <code class="literal">de-x-icu</code>.)
</p></dd><dt><span class="term"><code class="literal">und-x-icu</code> (for <span class="quote">“<span class="quote">undefined</span>”</span>)</span></dt><dd><p>
ICU <span class="quote">“<span class="quote">root</span>”</span> collation. Use this to get a reasonable
language-agnostic sort order.
</p></dd></dl></div><p>
</p><p>
Some (less frequently used) encodings are not supported by ICU. When the
database encoding is one of these, ICU collation entries
in <code class="literal">pg_collation</code> are ignored. Attempting to use one
will draw an error along the lines of <span class="quote">“<span class="quote">collation "de-x-icu" for
encoding "WIN874" does not exist</span>”</span>.
</p></div></div><div class="sect3" id="COLLATION-CREATE"><div class="titlepage"><div><div><h4 class="title">24.2.2.3. Creating New Collation Objects</h4></div></div></div><p>
If the standard and predefined collations are not sufficient, users can
create their own collation objects using the SQL
command <a class="xref" href="sql-createcollation.html" title="CREATE COLLATION"><span class="refentrytitle">CREATE COLLATION</span></a>.
</p><p>
The standard and predefined collations are in the
schema <code class="literal">pg_catalog</code>, like all predefined objects.
User-defined collations should be created in user schemas. This also
ensures that they are saved by <code class="command">pg_dump</code>.
</p><div class="sect4" id="id-1.6.11.4.5.7.4"><div class="titlepage"><div><div><h5 class="title">24.2.2.3.1. libc Collations</h5></div></div></div><p>
New libc collations can be created like this:
</p><pre class="programlisting">
CREATE COLLATION german (provider = libc, locale = 'de_DE');
</pre><p>
The exact values that are acceptable for the <code class="literal">locale</code>
clause in this command depend on the operating system. On Unix-like
systems, the command <code class="literal">locale -a</code> will show a list.
</p><p>
Since the predefined libc collations already include all collations
defined in the operating system when the database instance is
initialized, it is not often necessary to manually create new ones.
Reasons might be if a different naming system is desired (in which case
see also <a class="xref" href="collation.html#COLLATION-COPY" title="24.2.2.3.3. Copying Collations">Section 24.2.2.3.3</a>) or if the operating system has
been upgraded to provide new locale definitions (in which case see
also <a class="link" href="functions-admin.html#FUNCTIONS-ADMIN-COLLATION" title="Table 9.96. Collation Management Functions"><code class="function">pg_import_system_collations()</code></a>).
</p></div><div class="sect4" id="id-1.6.11.4.5.7.5"><div class="titlepage"><div><div><h5 class="title">24.2.2.3.2. ICU Collations</h5></div></div></div><p>
ICU allows collations to be customized beyond the basic language+country
set that is preloaded by <code class="command">initdb</code>. Users are encouraged
to define their own collation objects that make use of these facilities to
suit the sorting behavior to their requirements.
See <a class="ulink" href="https://unicode-org.github.io/icu/userguide/locale/" target="_top">https://unicode-org.github.io/icu/userguide/locale/</a>
and <a class="ulink" href="https://unicode-org.github.io/icu/userguide/collation/api.html" target="_top">https://unicode-org.github.io/icu/userguide/collation/api.html</a> for
information on ICU locale naming. The set of acceptable names and
attributes depends on the particular ICU version.
</p><p>
Here are some examples:
</p><div class="variablelist"><dl class="variablelist"><dt><span class="term"><code class="literal">CREATE COLLATION "de-u-co-phonebk-x-icu" (provider = icu, locale = 'de-u-co-phonebk');</code><br /></span><span class="term"><code class="literal">CREATE COLLATION "de-u-co-phonebk-x-icu" (provider = icu, locale = 'de@collation=phonebook');</code></span></dt><dd><p>German collation with phone book collation type</p><p>
The first example selects the ICU locale using a <span class="quote">“<span class="quote">language
tag</span>”</span> per BCP 47. The second example uses the traditional
ICU-specific locale syntax. The first style is preferred going
forward, but it is not supported by older ICU versions.
</p><p>
Note that you can name the collation objects in the SQL environment
anything you want. In this example, we follow the naming style that
the predefined collations use, which in turn also follow BCP 47, but
that is not required for user-defined collations.
</p></dd><dt><span class="term"><code class="literal">CREATE COLLATION "und-u-co-emoji-x-icu" (provider = icu, locale = 'und-u-co-emoji');</code><br /></span><span class="term"><code class="literal">CREATE COLLATION "und-u-co-emoji-x-icu" (provider = icu, locale = '@collation=emoji');</code></span></dt><dd><p>
Root collation with Emoji collation type, per Unicode Technical Standard #51
</p><p>
Observe how in the traditional ICU locale naming system, the root
locale is selected by an empty string.
</p></dd><dt><span class="term"><code class="literal">CREATE COLLATION latinlast (provider = icu, locale = 'en-u-kr-grek-latn');</code><br /></span><span class="term"><code class="literal">CREATE COLLATION latinlast (provider = icu, locale = 'en@colReorder=grek-latn');</code></span></dt><dd><p>
Sort Greek letters before Latin ones. (The default is Latin before Greek.)
</p></dd><dt><span class="term"><code class="literal">CREATE COLLATION upperfirst (provider = icu, locale = 'en-u-kf-upper');</code><br /></span><span class="term"><code class="literal">CREATE COLLATION upperfirst (provider = icu, locale = 'en@colCaseFirst=upper');</code></span></dt><dd><p>
Sort upper-case letters before lower-case letters. (The default is
lower-case letters first.)
</p></dd><dt><span class="term"><code class="literal">CREATE COLLATION special (provider = icu, locale = 'en-u-kf-upper-kr-grek-latn');</code><br /></span><span class="term"><code class="literal">CREATE COLLATION special (provider = icu, locale = 'en@colCaseFirst=upper;colReorder=grek-latn');</code></span></dt><dd><p>
Combines both of the above options.
</p></dd><dt><span class="term"><code class="literal">CREATE COLLATION numeric (provider = icu, locale = 'en-u-kn-true');</code><br /></span><span class="term"><code class="literal">CREATE COLLATION numeric (provider = icu, locale = 'en@colNumeric=yes');</code></span></dt><dd><p>
Numeric ordering, sorts sequences of digits by their numeric value,
for example: <code class="literal">A-21</code> < <code class="literal">A-123</code>
(also known as natural sort).
</p></dd></dl></div><p>
See <a class="ulink" href="https://www.unicode.org/reports/tr35/tr35-collation.html" target="_top">Unicode
Technical Standard #35</a>
and <a class="ulink" href="https://www.rfc-editor.org/info/bcp47" target="_top">BCP 47</a> for
details. The list of possible collation types (<code class="literal">co</code>
subtag) can be found in
the <a class="ulink" href="https://github.com/unicode-org/cldr/blob/master/common/bcp47/collation.xml" target="_top">CLDR
repository</a>.
</p><p>
Note that while this system allows creating collations that <span class="quote">“<span class="quote">ignore
case</span>”</span> or <span class="quote">“<span class="quote">ignore accents</span>”</span> or similar (using the
<code class="literal">ks</code> key), in order for such collations to act in a
truly case- or accent-insensitive manner, they also need to be declared as not
<em class="firstterm">deterministic</em> in <code class="command">CREATE COLLATION</code>;
see <a class="xref" href="collation.html#COLLATION-NONDETERMINISTIC" title="24.2.2.4. Nondeterministic Collations">Section 24.2.2.4</a>.
Otherwise, any strings that compare equal according to the collation but
are not byte-wise equal will be sorted according to their byte values.
</p><div class="note"><h3 class="title">Note</h3><p>
By design, ICU will accept almost any string as a locale name and match
it to the closest locale it can provide, using the fallback procedure
described in its documentation. Thus, there will be no direct feedback
if a collation specification is composed using features that the given
ICU installation does not actually support. It is therefore recommended
to create application-level test cases to check that the collation
definitions satisfy one's requirements.
</p></div></div><div class="sect4" id="COLLATION-COPY"><div class="titlepage"><div><div><h5 class="title">24.2.2.3.3. Copying Collations</h5></div></div></div><p>
The command <a class="xref" href="sql-createcollation.html" title="CREATE COLLATION"><span class="refentrytitle">CREATE COLLATION</span></a> can also be used to
create a new collation from an existing collation, which can be useful to
be able to use operating-system-independent collation names in
applications, create compatibility names, or use an ICU-provided collation
under a more readable name. For example:
</p><pre class="programlisting">
CREATE COLLATION german FROM "de_DE";
CREATE COLLATION french FROM "fr-x-icu";
</pre><p>
</p></div></div><div class="sect3" id="COLLATION-NONDETERMINISTIC"><div class="titlepage"><div><div><h4 class="title">24.2.2.4. Nondeterministic Collations</h4></div></div></div><p>
A collation is either <em class="firstterm">deterministic</em> or
<em class="firstterm">nondeterministic</em>. A deterministic collation uses
deterministic comparisons, which means that it considers strings to be
equal only if they consist of the same byte sequence. Nondeterministic
comparison may determine strings to be equal even if they consist of
different bytes. Typical situations include case-insensitive comparison,
accent-insensitive comparison, as well as comparison of strings in
different Unicode normal forms. It is up to the collation provider to
actually implement such insensitive comparisons; the deterministic flag
only determines whether ties are to be broken using bytewise comparison.
See also <a class="ulink" href="https://www.unicode.org/reports/tr10" target="_top">Unicode Technical
Standard 10</a> for more information on the terminology.
</p><p>
To create a nondeterministic collation, specify the property
<code class="literal">deterministic = false</code> to <code class="command">CREATE
COLLATION</code>, for example:
</p><pre class="programlisting">
CREATE COLLATION ndcoll (provider = icu, locale = 'und', deterministic = false);
</pre><p>
This example would use the standard Unicode collation in a
nondeterministic way. In particular, this would allow strings in
different normal forms to be compared correctly. More interesting
examples make use of the ICU customization facilities explained above.
For example:
</p><pre class="programlisting">
CREATE COLLATION case_insensitive (provider = icu, locale = 'und-u-ks-level2', deterministic = false);
CREATE COLLATION ignore_accents (provider = icu, locale = 'und-u-ks-level1-kc-true', deterministic = false);
</pre><p>
</p><p>
All standard and predefined collations are deterministic, all
user-defined collations are deterministic by default. While
nondeterministic collations give a more <span class="quote">“<span class="quote">correct</span>”</span> behavior,
especially when considering the full power of Unicode and its many
special cases, they also have some drawbacks. Foremost, their use leads
to a performance penalty. Note, in particular, that B-tree cannot use
deduplication with indexes that use a nondeterministic collation. Also,
certain operations are not possible with nondeterministic collations,
such as pattern matching operations. Therefore, they should be used
only in cases where they are specifically wanted.
</p><div class="tip"><h3 class="title">Tip</h3><p>
To deal with text in different Unicode normalization forms, it is also
an option to use the functions/expressions
<code class="function">normalize</code> and <code class="literal">is normalized</code> to
preprocess or check the strings, instead of using nondeterministic
collations. There are different trade-offs for each approach.
</p></div></div></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="locale.html" title="24.1. Locale Support">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="charset.html" title="Chapter 24. Localization">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="multibyte.html" title="24.3. Character Set Support">Next</a></td></tr><tr><td width="40%" align="left" valign="top">24.1. Locale Support </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 15.7 Documentation">Home</a></td><td width="40%" align="right" valign="top"> 24.3. Character Set Support</td></tr></table></div></body></html>
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