<?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>23.2. 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="23.1. Locale Support" /><link rel="next" href="multibyte.html" title="23.3. Character Set Support" /></head><body><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">23.2. Collation Support</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="locale.html" title="23.1. Locale Support">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="charset.html" title="Chapter 23. Localization">Up</a></td><th width="60%" align="center">Chapter 23. Localization</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 11.5 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="multibyte.html" title="23.3. Character Set Support">Next</a></td></tr></table><hr></hr></div><div class="sect1" id="COLLATION"><div class="titlepage"><div><div><h2 class="title" style="clear: both">23.2. Collation Support</h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="sect2"><a href="collation.html#id-1.6.10.4.4">23.2.1. Concepts</a></span></dt><dt><span class="sect2"><a href="collation.html#COLLATION-MANAGING">23.2.2. Managing Collations</a></span></dt></dl></div><a id="id-1.6.10.4.2" 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-1.6.10.4.4"><div class="titlepage"><div><div><h3 class="title">23.2.1. 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 domain 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">23.2.2. 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 that most tools provided by the operating system use. Another provider is <code class="literal">icu</code>, which uses the external ICU<a id="id-1.6.10.4.5.2.4" 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="23.3. Character Set Support">Section 23.3</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-1.6.10.4.5.5"><div class="titlepage"><div><div><h4 class="title">23.2.2.1. 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-1.6.10.4.5.6"><div class="titlepage"><div><div><h4 class="title">23.2.2.2. 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-1.6.10.4.5.6.4"><div class="titlepage"><div><div><h5 class="title">23.2.2.2.1. 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 9.86. 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 less 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-1.6.10.4.5.6.5"><div class="titlepage"><div><div><h5 class="title">23.2.2.2.2. 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">23.2.2.3. 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-1.6.10.4.5.7.4"><div class="titlepage"><div><div><h5 class="title">23.2.2.3.1. 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="23.2.2.3.3. Copying Collations">Section 23.2.2.3.3</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 9.86. Collation Management Functions"><code class="function">pg_import_system_collations()</code></a>). </p></div><div class="sect4" id="id-1.6.10.4.5.7.5"><div class="titlepage"><div><div><h5 class="title">23.2.2.3.2. 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="http://userguide.icu-project.org/locale" target="_top">http://userguide.icu-project.org/locale</a> and <a class="ulink" href="http://userguide.icu-project.org/collation/api" target="_top">http://userguide.icu-project.org/collation/api</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 digitslast (provider = icu, locale = 'en-u-kr-latn-digit');</code><br /></span><span class="term"><code class="literal">CREATE COLLATION digitslast (provider = icu, locale = 'en@colReorder=latn-digit');</code></span></dt><dd><p> Sort digits after Latin letters. (The default is digits before letters.) </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-latn-digit');</code><br /></span><span class="term"><code class="literal">CREATE COLLATION special (provider = icu, locale = 'en@colCaseFirst=upper;colReorder=latn-digit');</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="http://unicode.org/reports/tr35/tr35-collation.html" target="_top">Unicode Technical Standard #35</a> and <a class="ulink" href="https://tools.ietf.org/html/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="http://www.unicode.org/repos/cldr/trunk/common/bcp47/collation.xml" target="_top">CLDR repository</a>. The <a class="ulink" href="https://ssl.icu-project.org/icu-bin/locexp" target="_top">ICU Locale Explorer</a> can be used to check the details of a particular locale definition. The examples using the <code class="literal">k*</code> subtags require at least ICU version 54. </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), PostgreSQL does not at the moment allow such collations to act in a truly case- or accent-insensitive manner. 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">23.2.2.3.3. 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></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="locale.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="charset.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="multibyte.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">23.1. Locale Support </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> 23.3. Character Set Support</td></tr></table></div></body></html>