More than one function may be defined with the same name, so long as the arguments they take are different. In other words, function names can be overloaded. 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.
As of Postgres 7.0, 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. This is now the preferred technique to accomplish function overloading.
For functions written in C, the SQL name declared in CREATE FUNCTION must be exactly the same as the actual name of the function in the C code (hence it must be a legal C function name).
There is a subtle implication of this restriction: while the dynamic loading routines in most operating systems are more than happy to allow you to load any number of shared libraries that contain conflicting (identically-named) function names, they may in fact botch the load in interesting ways. For example, if you define a dynamically-loaded function that happens to have the same name as a function built into Postgres, the DEC OSF/1 dynamic loader causes Postgres to call the function within itself rather than allowing Postgres to call your function. Hence, if you want your function to be used on different architectures, we recommend that you do not overload C function names.
There is a clever trick to get around the problem just described. Since there is no problem overloading SQL functions, you can define 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.
Another solution is not to use dynamic loading, but to link your functions into the backend statically and declare them as INTERNAL functions. Then, the functions must all have distinct C names but they can be declared with the same SQL names (as long as their argument types differ, of course). This way avoids the overhead of an SQL wrapper function, at the cost of more effort to prepare a custom backend executable. (This option is only available in version 6.5 and later, since prior versions required internal functions to have the same name in SQL as in the C code.)
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