SGML source for new documentation.

doc/src/sgml/xoper.sgml

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+<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>