A function-based index is an index based on the return values of an expression or function. The function may be a method function of an object type.

A function-based index built on a method function precomputes the return value of the function for each object instance in the column or table being indexed and stores those values in the index. There they can be referenced without having to evaluate the function again.

Function-based indexes are useful for improving the performance of queries that have a function in the WHERE clause. For example, the following code contains a query of an object table emps:

To evaluate this query, Oracle must evaluate bonus() for each row object in the table. If there is a function-based index on the return values of bonus(), then this work has already been done, and Oracle can simply look up the results in the index. This enables Oracle to return a result from the query more quickly.

Return values of a function can be usefully indexed only if those values are constant, that is, only if the function always returns the same value for each object instance. For this reason, to use a user-written function in a function-based index, the function must have been declared with the DETERMINISTIC keyword, as in the preceding example. This keyword promises that the function always returns the same value for each object instance's set of input argument values.

The following example creates a function-based index on the method bonus() in the table emps:

Example 9-14 Creating a Function-Based Index on a Method