<Chapter>
<Title>Extending <Acronym>SQL</Acronym>: Operators</Title>

<Para>
     <ProductName>Postgres</ProductName> supports left unary, right  unary  and  binary
     operators.   Operators  can  be  overloaded, or re-used
     with different numbers  and  types  of  arguments.   If
     there  is  an ambiguous situation and the system cannot
     determine the correct operator to use, it  will  return
     an  error  and you may have to typecast the left and/or
     right operands to help it understand which operator you
     meant to use.
     To  create  an  operator for adding two complex numbers
     can be done as follows.  First  we  need  to  create  a
     function  to add the new types. Then, we can create the
     operator with the function.

<ProgramListing>
         CREATE FUNCTION complex_add(complex, complex)
            RETURNS complex
            AS '$PWD/obj/complex.so'
            LANGUAGE 'c';

         CREATE OPERATOR + (
            leftarg = complex,
            rightarg = complex,
            procedure = complex_add,
            commutator = +
         );
</ProgramListing>
</Para>

<Para>
     We've shown how to create a binary  operator  here.  To
     create  unary  operators, just omit one of leftarg (for
     left unary) or rightarg (for right unary).
     If we give the system enough type information,  it  can
     automatically figure out which operators to use.
     
<ProgramListing>
         SELECT (a + b) AS c FROM test_complex;

         +----------------+
         |c               |
         +----------------+
         |(5.2,6.05)      |
         +----------------+
         |(133.42,144.95) |
         +----------------+
</ProgramListing>
</Para>
</Chapter>
