More than one function may 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.
A function may also have the same name as an attribute. In the case that there is an ambiguity between a function on a complex type and an attribute of the complex type, the attribute will always be used.
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 test(1, 1.5). The currently implemented resolution rules are described in the User's Guide, but it is unwise to design a system that subtly relies on this behavior.
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. E.g.,
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.
Prior to PostgreSQL 7.0, this alternative syntax did not exist. There is a trick to get around the problem, by defining a set of C functions with different names and then define a set of identically-named SQL function wrappers that take the appropriate argument types and call the matching C function.