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Function-based Indexes: boost up the Performance

Using Function-based Indexes for Performance
A function-based index includes columns that are either transformed by a function, such as the UPPER function, or included in an expression, such as col1 + col2. With a function-based index, you can store computation-intensive expressions in the index.

Defining a function-based index on the transformed column or expression allows that data to be returned using the index when that function or expression is used in a WHERE clause or an ORDER BY clause. This allows Oracle to bypass computing the value of the expression when processing SELECT and DELETE statements. Therefore, a function-based index can be beneficial when frequently-executed SQL statements include transformed columns, or columns in expressions, in a WHERE or ORDER BY clause.

Oracle treats descending indexes as function-based indexes. The columns marked DESC are sorted in descending order.

For example, function-based indexes defined with the UPPER(column_name) or LOWER(column_name) keywords allow case-insensitive searches. The index created in the following statement:

CREATE INDEX uppercase_idx ON employees (UPPER(last_name));

facilitates processing queries such as:

SELECT * FROM employees
WHERE UPPER(last_name) = 'MARKSON';

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