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Diffstat (limited to 'doc/src/sgml/charset.sgml')
| -rw-r--r-- | doc/src/sgml/charset.sgml | 61 |
1 files changed, 56 insertions, 5 deletions
diff --git a/doc/src/sgml/charset.sgml b/doc/src/sgml/charset.sgml index a6143ef8a74..555d1b4ac63 100644 --- a/doc/src/sgml/charset.sgml +++ b/doc/src/sgml/charset.sgml @@ -847,11 +847,13 @@ CREATE COLLATION german (provider = libc, locale = 'de_DE'); <para> Note that while this system allows creating collations that <quote>ignore - case</quote> or <quote>ignore accents</quote> or similar (using - the <literal>ks</literal> 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. + case</quote> or <quote>ignore accents</quote> or similar (using the + <literal>ks</literal> key), in order for such collations to act in a + truly case- or accent-insensitive manner, they also need to be declared as not + <firstterm>deterministic</firstterm> in <command>CREATE COLLATION</command>; + see <xref linkend="collation-nondeterministic"/>. + Otherwise, any strings that compare equal according to the collation but + are not byte-wise equal will be sorted according to their byte values. </para> <note> @@ -883,6 +885,55 @@ CREATE COLLATION french FROM "fr-x-icu"; </para> </sect4> </sect3> + + <sect3 id="collation-nondeterministic"> + <title>Nondeterminstic Collations</title> + + <para> + A collation is either <firstterm>deterministic</firstterm> or + <firstterm>nondeterministic</firstterm>. 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 comparion 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 <ulink url="https://unicode.org/reports/tr10">Unicode Technical + Standard 10</ulink> for more information on the terminology. + </para> + + <para> + To create a nondeterministic collation, specify the property + <literal>deterministic = false</literal> to <command>CREATE + COLLATION</command>, for example: +<programlisting> +CREATE COLLATION ndcoll (provider = icu, locale = 'und', deterministic = false); +</programlisting> + 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: +<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); +</programlisting> + </para> + + <para> + All standard and predefined collations are deterministic, all + user-defined collations are deterministic by default. While + nondeterministic collations give a more <quote>correct</quote> 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. 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. + </para> + </sect3> </sect2> </sect1> |