posted 20 years ago
Sajee,
I know that several DBMSs -- including Oracle and IBM DB2 (latest versions) -- contain "query optimizers". Depending on the way they are configured, they may rewrite the query you enter, in order to maximize its efficiency. Sometimes, it is more efficient for a query to do a "full table scan" than to use the index. Hence, it is quite reasonable to see a query plan that does not use indexes.
Some people say that if a query will return at least 15% of a table's entire rows, then it should not use the index.
Hope this has helped you.
Good Luck,
Avi.