Indexes generally improve the performance of queries and DML statements that operate on a single, existing row or a small number of existing rows. However, too many indexes can increase the processing overhead for statements that add, modify, or delete rows.
Before you add additional indexes, examine the performance of your database for queries and DML. You can then compare performance after the new indexes are added.
See Also:
Oracle Database Testing Guide for information about using the SQL Performance Analyzer to analyze the SQL performance impact of any type of schema or system changes