An index (also known as a secondary key) is a database object that can be defined for an individual column or a series of columns in a database table. An index consists of one or more inversion lists.
Since indexes are derived entirely from data already in the database, they can be deleted without data being lost.
An index name must be unique when combined with the table name. When defining an index, the user determines whether the column values of different rows in the indexed columns have to be unique or not.
Indexes speed up access to the rows in a database table. However, all changes to data must also be maintained in the relevant index, which itself takes time.
See also:
CREATE INDEX Statement (create_index_statement)
Database Manager GUI, Restoring the Indexes After a Database Recovery
Database Manager CLI, Recreating a Damaged Index