37.6. Function Overloading
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 . Whether or not you use it, this capability entails security precautions when calling functions in databases where some users mistrust other users; see Section 10.3 . 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 functions:
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
currently implemented resolution rules are described in
, 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.
Another possible conflict is between variadic and non-variadic functions.
For instance, it is possible to create both
. In this case it is unclear which one
should be matched to a call providing a single numeric argument, such as
. 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
clause for the SQL
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' LANGUAGE C; CREATE FUNCTION test(int, int) RETURNS int AS '
filename', 'test_2arg' LANGUAGE C;
The names of the C functions here reflect one of many possible conventions.