Invoking a Function That Is Bound to an Operator

Functions that are bound to specific operators get invoked automatically without explicit invocation. Suppose an equal() function exists that takes two arguments of type1 and returns a Boolean. If the equal operator (=) is used for comparisons between two columns, col_1 and col_2, that are of type1, the equal() function is invoked automatically. For example, the following query implicitly invokes the appropriate equal() function to evaluate the WHERE clause:

SELECT * FROM tab_1 
WHERE col_1 = col_2

The preceding query evaluates as though it had been specified as follows:

SELECT * FROM tab_1 
WHERE equal (col_1, col_2)