michael_bialik's blog

Indexes are used to speed up data access by SQL statements, but there is no free lunch as each additional index increases:

• The time needed to perform DML (Insert/Update/Delete) operation on the table (because additional index entries must be updated).
• The enqueue time (during DML the corresponding index entries are locked decreasing the ability of parallel updates and causing transactions, issued by another session(s) to wait.
• The generated UNDO volume.
• The disk space needed to store the index information.