An index is an optional data structure used to speed retrieval of data or enforce certain constraints (such as uniqueness) on the records in a table. When you create an index on a column (or columns), the XDB Server stores an appropriately ordered list of all the values in that column as a separate index file. While this can greatly improve the performance of queries, maintaining an index slows down DML update operations (such as INSERT and DELETE) somewhat, and the index itself takes up memory. You must make a decision about creating indexes each time you create a table.