This is pretty minimal but covers the bare facts.
<sect1 id="collation">
<title>Collation Support</title>
+ <indexterm zone="collation"><primary>collation</></>
+
<para>
The collation feature allows specifying the sort order and certain
other locale aspects of data per-column, or even per-operation.
structure on-the-fly.
</para>
</sect2>
+
+ <sect2 id="plpgsql-declaration-collation">
+ <title>Collation of <application>PL/pgSQL</application> Variables</title>
+
+ <indexterm>
+ <primary>collation</>
+ <secondary>in PL/pgSQL</>
+ </indexterm>
+
+ <para>
+ When a <application>PL/pgSQL</application> function has one or more
+ parameters of collatable data types, a collation is identified for each
+ function call depending on the collations assigned to the actual
+ arguments, as described in <xref linkend="collation">. If a collation is
+ successfully identified (i.e., there are no conflicts of implicit
+ collations among the arguments) then all the collatable parameters are
+ treated as having that collation implicitly. This will affect the
+ behavior of collation-sensitive operations within the function.
+ For example, consider
+
+<programlisting>
+CREATE FUNCTION less_than(a text, b text) RETURNS boolean AS $$
+BEGIN
+ RETURN a < b;
+END;
+$$ LANGUAGE plpgsql;
+
+SELECT less_than(text_field_1, text_field_2) FROM table1;
+SELECT less_than(text_field_1, text_field_2 COLLATE "C") FROM table1;
+</programlisting>
+
+ The first use of <function>less_than</> will use the common collation
+ of <structfield>text_field_1</> and <structfield>text_field_2</> for
+ the comparison, while the second use will use <literal>C</> collation.
+ </para>
+
+ <para>
+ Furthermore, the identified collation is also assumed as the collation of
+ any local variables that are of collatable types. Thus this function
+ would not work any differently if it were written as
+
+<programlisting>
+CREATE FUNCTION less_than(a text, b text) RETURNS boolean AS $$
+DECLARE
+ local_a text := a;
+ local_b text := b;
+BEGIN
+ RETURN local_a < local_b;
+END;
+$$ LANGUAGE plpgsql;
+</programlisting>
+ </para>
+
+ <para>
+ If there are no parameters of collatable data types, or no common
+ collation can be identified for them, then parameters and local variables
+ use the default collation of their data type (which is usually the
+ database's default collation, but could be different for variables of
+ domain types).
+ </para>
+
+ <para>
+ Explicit <literal>COLLATE</> clauses can be written inside a function
+ if it is desired to force a particular collation to be used regardless
+ of what the function is called with. For example,
+
+<programlisting>
+CREATE FUNCTION less_than_c(a text, b text) RETURNS boolean AS $$
+BEGIN
+ RETURN a < b COLLATE "C";
+END;
+$$ LANGUAGE plpgsql;
+</programlisting>
+ </para>
+ </sect2>
</sect1>
<sect1 id="plpgsql-expressions">
</screen>
</para>
</sect2>
+
+ <sect2>
+ <title><acronym>SQL</acronym> Functions with Collations</title>
+
+ <indexterm>
+ <primary>collation</>
+ <secondary>in SQL functions</>
+ </indexterm>
+
+ <para>
+ When a SQL function has one or more parameters of collatable data types,
+ a collation is identified for each function call depending on the
+ collations assigned to the actual arguments, as described in <xref
+ linkend="collation">. If a collation is successfully identified
+ (i.e., there are no conflicts of implicit collations among the arguments)
+ then all the collatable parameters are treated as having that collation
+ implicitly. This will affect the behavior of collation-sensitive
+ operations within the function. For example, using the
+ <function>anyleast</> function described above, the result of
+<programlisting>
+SELECT anyleast('abc'::text, 'ABC');
+</programlisting>
+ will depend on the database's default collation. In <literal>C</> locale
+ the result will be <literal>ABC</>, but in many other locales it will
+ be <literal>abc</>. The collation to use can be forced by adding
+ a <literal>COLLATE</> clause to any of the arguments, for example
+<programlisting>
+SELECT anyleast('abc'::text, 'ABC' COLLATE "C");
+</programlisting>
+ Alternatively, if you wish a function to operate with a particular
+ collation regardless of what it is called with, insert
+ <literal>COLLATE</> clauses as needed in the function definition.
+ This version of <function>anyleast</> would always use <literal>en_US</>
+ locale to compare strings:
+<programlisting>
+CREATE FUNCTION anyleast (VARIADIC anyarray) RETURNS anyelement AS $$
+ SELECT min($1[i] COLLATE "en_US") FROM generate_subscripts($1, 1) g(i);
+$$ LANGUAGE SQL;
+</programlisting>
+ But note that this will throw an error if applied to a non-collatable
+ data type.
+ </para>
+
+ <para>
+ If no common collation can be identified among the actual arguments,
+ then a SQL function treats its parameters as having their data types'
+ default collation (which is usually the database's default collation,
+ but could be different for parameters of domain types).
+ </para>
+
+ <para>
+ The behavior of collatable parameters can be thought of as a limited
+ form of polymorphism, applicable only to textual data types.
+ </para>
+ </sect2>
</sect1>
<sect1 id="xfunc-overload">
projects / postgresql / commitdiff