You read about the main purposes of an index in “Determining what columns to index” on page 105. Indexes provide efficient access to data. When you create a table that contains a primary key, you must create a unique index for that table on the primary key. DB2 marks the table as unavailable until the explicit creation of the required indexes.
You can also choose to use indexes because of access requirements.
Be aware that using indexes involves a tradeoff. A greater number of indexes can simultaneously improve the performance of a particular transaction's access and require additional processing for inserting, updating, and deleting index keys.
After you create an index, DB2 maintains the index, but you can perform necessary ...