An index provides an ordering on the rows in a column or columns of a table. An index is like a telephone book that initially sorts people by surname, and then sorts identical surnames by first names. This ordering speeds up searches for phone numbers for a particular surname, but it does not provide help in finding the phone number at a particular address. In the same way, a database index is useful only for searches on a specific column or columns.
Indexes get more useful as the size of the table increases. The average time to find a phone number at a given address increases with the size of the phone book, while it does not take much longer to find the phone number of K. Kaminski in a large phone book than in a small phone book.
The database server query optimizer automatically uses an index when a suitable index exists and when using one will improve performance.
There are some down sides to creating indexes. In particular, any indexes must be maintained along with the table itself when the data in a column is modified, so that the performance of inserts, updates, and deletes can be affected by indexes. For this reason, unnecessary indexes should be dropped. Use the Index Consultant to identify unnecessary indexes. See Using the Index Consultant.
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