Index on query
- The table and index are distinct data structures.
- Indexes are never referred to in DML statements.
- An index is used in evaluating a query if the optimizer
decides to use it.
- Indexes affect performance only. No impact on query results.
Index on update
- Index maintenance is automatic. E.g.,
- Updates the row,
- Deletes the index entry for the old key,
- And adds an index entry for the new key.