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Documentation update for Standard Collations. 

Correct out-of-date text that said the "default" collation is always
based on LC_COLLATE and LC_CTYPE.

Also reformat into a list to make it easier to understand and compare
the available collations, and briefly document the stability
characteristics of each one.

Discussion: https://postgr.es/m/4a69d067374d2f6bfb66f5bfb2ab9a020493d49f.camel@j-davis.com
This commit is contained in:
Jeff Davis 2024-03-02 13:37:43 -08:00
parent 1e01374654
commit 875e46a0a2
1 changed files with 45 additions and 27 deletions
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doc/src/sgml/charset.sgml
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@ -788,37 +788,19 @@ SELECT * FROM test1 ORDER BY a || b COLLATE "fr_FR";
<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>
@ -828,15 +810,51 @@ SELECT * FROM test1 ORDER BY a || b COLLATE "fr_FR";
<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">