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 overloaded. 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.
When creating a family of overloaded functions, one should be
careful not to create ambiguities. For instance, given the
CREATE FUNCTION test(int, real) RETURNS ...
CREATE FUNCTION test(smallint, double precision) RETURNS ...
it is not immediately clear which function would be called with
some trivial input like test(1, 1.5). The
currently implemented resolution rules are described in
Chapter 10, but it is unwise to design a system that subtly
relies on this behavior.
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 attribute(table) is considered equivalent
to table.attribute. 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, schema.func(table)) but it's better to
avoid the problem by not choosing conflicting names.
Another possible conflict is between variadic and non-variadic functions.
For instance, it is possible to create both foo(numeric) and
foo(VARIADIC numeric). In this case it is unclear which one
should be matched to a call providing a single numeric argument, such as
foo(10.1). 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.
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
AS clause for the SQL CREATE
FUNCTION command decouples the SQL function name from
the function name in the C source code. For instance:
CREATE FUNCTION test(int) RETURNS int
AS 'filename', 'test_1arg'
CREATE FUNCTION test(int, int) RETURNS int
AS 'filename', 'test_2arg'
The names of the C functions here reflect one of many possible conventions.