<title>Standard Collations</title>

<para>

On all platforms, the collations named <literal>default</literal>,

<literal>C</literal>, and <literal>POSIX</literal> are available. Additional

collations may be available depending on operating system support.

The <literal>default</literal> collation selects the <symbol>LC_COLLATE</symbol>

and <symbol>LC_CTYPE</symbol> values specified at database creation time.

The <literal>C</literal> and <literal>POSIX</literal> collations both specify

<quote>traditional C</quote> behavior, in which only the ASCII letters

<quote><literal>A</literal></quote> through <quote><literal>Z</literal></quote>

are treated as letters, and sorting is done strictly by character

code byte values.

</para>

<note>

<para>

The <literal>C</literal> and <literal>POSIX</literal> locales may behave

differently depending on the database encoding.

</para>

</note>

<para>

Additionally, two SQL standard collation names are available:

On all platforms, the following collations are supported:

<variablelist>

<varlistentry>

<term><literal>unicode</literal></term>

<listitem>

<para>

This collation sorts using the Unicode Collation Algorithm with the

Default Unicode Collation Element Table. It is available in all

encodings. ICU support is required to use this collation. (This

collation has the same behavior as the ICU root locale; see <xref

This SQL standard collation sorts using the Unicode Collation

Algorithm with the Default Unicode Collation Element Table. It is

available in all encodings. ICU support is required to use this

collation, and behavior may change if Postgres is built with a

different version of ICU. (This collation has the same behavior as

the ICU root locale; see <xref

linkend="collation-managing-predefined-icu-und-x-icu"/>.)

</para>

</listitem>

<term><literal>ucs_basic</literal></term>

<listitem>

<para>

This collation sorts by Unicode code point. It is only available for

encoding <literal>UTF8</literal>. (This collation has the same

This SQL standard collation sorts using the Unicode code point values

rather than natural language order, and only the ASCII letters

<quote><literal>A</literal></quote> through

<quote><literal>Z</literal></quote> are treated as letters. The

behavior is efficient and stable across all versions. Only available

for encoding <literal>UTF8</literal>. (This collation has the same

behavior as the libc locale specification <literal>C</literal> in

<literal>UTF8</literal> encoding.)

</para>

</listitem>

</varlistentry>

<varlistentry>

<term><literal>C</literal> (equivalent to <literal>POSIX</literal>)</term>

<listitem>

<para>

The <literal>C</literal> and <literal>POSIX</literal> collations are

based on <quote>traditional C</quote> behavior. They sort by byte

values rather than natural language order, and only the ASCII letters

<quote><literal>A</literal></quote> through

<quote><literal>Z</literal></quote> are treated as letters. The

behavior is efficient and stable across all versions for a given

database encoding, but behavior may vary between different database

encodings.

</para>

</listitem>

</varlistentry>

<varlistentry>

<term><literal>default</literal></term>

<listitem>

<para>

The <literal>default</literal> collation selects the locale specified

at database creation time.

</para>

</listitem>

</varlistentry>

</variablelist>

</para>

<para>

Additional collations may be available depending on operating system

support. The efficiency and stability of these additional collations

depend on the collation provider, the provider version, and the locale.

</para>

</sect3>

<sect3 id="collation-managing-predefined">