<title>Function Overloading</title>

   <indexterm zone="xfunc-overload">
    <primary>overloading</primary>
    <secondary>functions</secondary>
   </indexterm>

   <para>
    More than one function can be defined with the same SQL name, so long
    as the arguments they take are different.  In other words,
    function names can be <firstterm>overloaded</firstterm>.  Whether or not
    you use it, this capability entails security precautions when calling
    functions in databases where some users mistrust other users; see
    <xref linkend="typeconv-func"/>.  When a query is executed, the server
    will determine which function to call from the data types and the number
    of the provided arguments.  Overloading can also be used to simulate
    functions with a variable number of arguments, up to a finite maximum
    number.
   </para>

   <para>
    When creating a family of overloaded functions, one should be
    careful not to create ambiguities.  For instance, given the
    functions:
<programlisting>
CREATE FUNCTION test(int, real) RETURNS ...
CREATE FUNCTION test(smallint, double precision) RETURNS ...
</programlisting>
    it is not immediately clear which function would be called with
    some trivial input like <literal>test(1, 1.5)</literal>.  The
    currently implemented resolution rules are described in
    <xref linkend="typeconv"/>, but it is unwise to design a system that subtly
    relies on this behavior.
   </para>

   <para>
    A function that takes a single argument of a composite type should
    generally not have the same name as any attribute (field) of that type.
    Recall that <literal><replaceable>attribute</replaceable>(<replaceable>table</replaceable>)</literal>
    is considered equivalent
    to <literal><replaceable>table</replaceable>.<replaceable>attribute</replaceable></literal>.
    In the case that there is an
    ambiguity between a function on a composite type and an attribute of
    the composite type, the attribute will always be used.  It is possible
    to override that choice by schema-qualifying the function name
    (that is, <literal><replaceable>schema</replaceable>.<replaceable>func</replaceable>(<replaceable>table</replaceable>)
    </literal>) but it's better to
    avoid the problem by not choosing conflicting names.
   </para>

   <para>
    Another possible conflict is between variadic and non-variadic functions.
    For instance, it is possible to create both <literal>foo(numeric)</literal> and
    <literal>foo(VARIADIC numeric[])</literal>.  In this case it is unclear which one
    should be matched to a call providing a single numeric argument, such as
    <literal>foo(10.1)</literal>.  The rule is that the function appearing
    earlier in the search path is used, or if the two functions are in the
    same schema, the non-variadic one is preferred.
   </para>

   <para>
    When overloading C-language functions, there is an additional
    constraint: The C name of each function in the family of
    overloaded functions must be different from the C names of all
    other functions, either internal or dynamically loaded.  If this
    rule is violated, the behavior is not portable.  You might get a
    run-time linker error, or one of the functions will get called
    (usually the internal one).  The alternative form of the
    <literal>AS</literal> clause for the SQL <command>CREATE
    FUNCTION</command> command decouples the SQL function name from
    the function name in the C source code.  For instance:
<programlisting>
CREATE FUNCTION test(int) RETURNS int
    AS '<replaceable>filename</replaceable>', 'test_1arg'
    LANGUAGE C;
CREATE FUNCTION test(int, int) RETURNS int
    AS '<replaceable>filename</replaceable>', 'test_2arg'
    LANGUAGE C;
</programlisting>
    The names of the C functions here reflect one of many possible conventions.
   </para>
  </sect1>

  <sect1 id="xfunc-volatility">
   <title>Function Volatility Categories</title>

   <indexterm zone="xfunc-volatility">