Every operator is “syntactic sugar” for a call to an underlying function that does the real work; so you must first create the underlying function before you can create the operator. However, an operator is not merely syntactic sugar, because it carries additional information that helps the query planner optimize queries that use the operator. The next section will be devoted to explaining that additional information.
PostgreSQL supports left unary, right unary, and binary operators. Operators can be overloaded; that is, the same operator name can be used for different operators that have different numbers and types of operands. When a query is executed, the system determines the operator to call from the number and types of the provided operands.
Here is an example of creating an operator for adding two complex numbers. We assume we've already created the definition of type
complex (see Section 38.12). First we need a function that does the work, then we can define the operator:
CREATE FUNCTION complex_add(complex, complex) RETURNS complex AS '
filename', 'complex_add' LANGUAGE C IMMUTABLE STRICT; CREATE OPERATOR + ( leftarg = complex, rightarg = complex, function = complex_add, commutator = + );
Now we could execute a query like this:
SELECT (a + b) AS c FROM test_complex; c ----------------- (5.2,6.05) (133.42,144.95)
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). The
function clause and the argument clauses are the only required items in
CREATE OPERATOR. The
commutator clause shown in the example is an optional hint to the query optimizer. Further details about
commutator and other optimizer hints appear in the next section.
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