from two arbitrary data type elements:
<screen>
CREATE FUNCTION make_array(anyelement, anyelement) RETURNS anyarray AS $$
    SELECT ARRAY[$1, $2];
$$ LANGUAGE SQL;

SELECT make_array(1, 2) AS intarray, make_array('a'::text, 'b') AS textarray;
 intarray | textarray
----------+-----------
 {1,2}    | {a,b}
(1 row)
</screen>
    </para>

    <para>
     Notice the use of the typecast <literal>'a'::text</literal>
     to specify that the argument is of type <type>text</type>. This is
     required if the argument is just a string literal, since otherwise
     it would be treated as type
     <type>unknown</type>, and array of <type>unknown</type> is not a valid
     type.
     Without the typecast, you will get errors like this:
<screen>
ERROR:  could not determine polymorphic type because input has type unknown
</screen>
    </para>

    <para>
     With <function>make_array</function> declared as above, you must
     provide two arguments that are of exactly the same data type; the
     system will not attempt to resolve any type differences.  Thus for
     example this does not work:
<screen>
SELECT make_array(1, 2.5) AS numericarray;
ERROR:  function make_array(integer, numeric) does not exist
</screen>
     An alternative approach is to use the <quote>common</quote> family of
     polymorphic types, which allows the system to try to identify a
     suitable common type:
<screen>
CREATE FUNCTION make_array2(anycompatible, anycompatible)
RETURNS anycompatiblearray AS $$
    SELECT ARRAY[$1, $2];
$$ LANGUAGE SQL;

SELECT make_array2(1, 2.5) AS numericarray;
 numericarray
--------------
 {1,2.5}
(1 row)
</screen>
     Because the rules for common type resolution default to choosing
     type <type>text</type> when all inputs are of unknown types, this
     also works:
<screen>
SELECT make_array2('a', 'b') AS textarray;
 textarray
-----------
 {a,b}
(1 row)
</screen>
    </para>

    <para>
     It is permitted to have polymorphic arguments with a fixed
     return type, but the converse is not. For example:
<screen>
CREATE FUNCTION is_greater(anyelement, anyelement) RETURNS boolean AS $$
    SELECT $1 &gt; $2;
$$ LANGUAGE SQL;

SELECT is_greater(1, 2);
 is_greater
------------
 f
(1 row)

CREATE FUNCTION invalid_func() RETURNS anyelement AS $$
    SELECT 1;
$$ LANGUAGE SQL;
ERROR:  cannot determine result data type
DETAIL:  A result of type anyelement requires at least one input of type anyelement, anyarray, anynonarray, anyenum, or anyrange.
</screen>
    </para>

    <para>
     Polymorphism can be used with functions that have output arguments.
     For example:
<screen>
CREATE FUNCTION dup (f1 anyelement, OUT f2 anyelement, OUT f3 anyarray)
AS 'select $1, array[$1,$1]' LANGUAGE SQL;

SELECT * FROM dup(22);
 f2 |   f3
----+---------
 22 | {22,22}
(1 row)
</screen>
    </para>

    <para>
     Polymorphism can also be used with variadic functions.
     For example:
<screen>
CREATE FUNCTION anyleast (VARIADIC anyarray) RETURNS anyelement AS $$
    SELECT min($1[i]) FROM generate_subscripts($1, 1) g(i);
$$ LANGUAGE SQL;

SELECT anyleast(10, -1, 5, 4);
 anyleast
----------
       -1
(1 row)

SELECT anyleast('abc'::text, 'def');
 anyleast
----------
 abc
(1 row)

CREATE FUNCTION concat_values(text, VARIADIC anyarray) RETURNS text AS $$
    SELECT array_to_string($2, $1);
$$ LANGUAGE SQL;

SELECT concat_values('|', 1, 4, 2);
 concat_values
---------------
 1|4|2
(1 row)
</screen>
    </para>
   </sect2>

   <sect2 id="xfunc-sql-collations">
    <title><acronym>SQL</acronym> Functions with Collations</title>

    <indexterm>
     <primary>collation</primary>
     <secondary>in SQL functions</secondary>
    </indexterm>

    <para>
     When an 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.,