PostgreSQL, Second Edition

Polymorphic Functions

Starting with PostgreSQL version 8.0, you can write polymorphic functions in PL/pgSQL. A polymorphic function is a function with at least one parameter of type ANYELEMENT or ANYARRAY. The types ANYELEMENT and ANYARRAY are called polymorphic types because they can assume different “shapes” at run-time.

Here's a simple polymorphic function that will return the greater of two arguments:

-- ch07.sql
CREATE OR REPLACE FUNCTION max( arg1 ANYELEMENT, arg2 ANYELEMENT ) 
    RETURNS ANYELEMENT AS $$
  BEGIN

    IF( arg1 > arg2 ) THEN
      RETURN( arg1 );
    ELSE
      RETURN( arg2 );
    END IF;

  END;
$$ LANGUAGE 'plpgsql';

When you call this function with two INTEGER values, PL/pgSQL treats the function as if you had defined it as

 CREATE OR REPLACE FUNCTION ...