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; however, adding indexes does slow down write operations, such as inserts, deletes, and updates on that table. For details on creating indexes to improve performance, read 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. A unique index is always created, when the UNIQUE restraint is used when creating an index.

You can create and delete indexes using the SQL statements.