James Harper <james(dot)harper(at)bendigoit(dot)com(dot)au> writes:
> is it possible to have a function that can return a different type
> depending on the parameters?
The data type of any expression (including a function call) has to be
determinable at parse time, so no you can't just randomly return a
run-time-determined data type.
However, have you looked at the "polymorphic functions" feature?
You can declare a function as returning the same data type that
one of its inputs has. This seems to cover most of the cases
that are useful in practice.
http://www.postgresql.org/docs/9.3/static/extend-type-system.html#EXTEND-TYPES-POLYMORPHIC
http://www.postgresql.org/docs/9.3/static/xfunc-sql.html#AEN52916
regards, tom lane