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Oracle Concepts - Function-based indexes

Oracle Tips by Burleson Consulting

Function-based indexes

One of the most important advances in Oracle indexing is the introduction of function-based indexing. Function-based indexes allow creation of indexes on expressions, internal functions, and user-written functions in PL/SQL and Java. Function-based indexes ensure that the Oracle designer is able to use an index for its query. Prior to Oracle8, the use of a built-in function would not be able to match the performance of an index. Consequently, Oracle would perform the dreaded full-table scan. Examples of SQL with function-based queries might include the following:

Select * from customer where substr(cust_name,1,4) = ?BURL?;

Select * from customer where to_char(order_date,?MM?) = ?01;

Select * from customer where upper(cust_name) = ?JONES?;

Select * from customer where initcap(first_name) = ?Mike?;

Oracle always interrogates the where clause of the SQL statement to see if a matching index exists. By using function-based indexes, the Oracle designer can create a matching index that exactly matches the predicates within the SQL where clause. This ensures that the query is retrieved with a minimal amount of disk I/O and the fastest possible speed.

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