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michael_bialik's blog

Removing duplicate indexes

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.