Indexes are used to speed up access to tables. The database engine uses indexes to access the rows in a table directly. Without indexes, the engine would have to search the whole contents of a table to find the desired row. You can create as many indexes as you like on a single table but adding indexes does slow down write operations, such as inserts, deletes, and updates on that table. For details about creating indexes to improve performance, see Using indexes to improve query performance.

There are two kinds of indexes: non-unique indexes and unique indexes. A unique index is an index where all key values are unique. To create a unique index, use the UNIQUE restraint when creating the index.

The following table describes the statements used to create and delete indexes.