DB2 Version 7.2 allows columns with a length greater than 255 bytes to be specified as part of an index key. A larger index key has a positive effect on the usefulness of an index. The DB2_INDEX_2BYTEVARLEN registry variable allows for the use of two bytes instead of one to store the length of an index key. To convert existing indexes to use larger index keys: drop the indexes, set the DB2_INDEX_2BYTEVARLEN registry variable to ON, then recreate the indexes (using the larger columns).
For more information on larger index keys, see the DB2 Release Notes.