The IMS interface modules DTLAMDL1 and DTLAMDBA use GU keyed access if the access supports the values supplied in the WHERE clause for all CCK columns .

The IMS interface modules uses non-optimized GN calls to retrieve records when one of the following is true:

The WHERE clause includes unsupported values for CCK columns.
The low- and high-rationalized key values differ because the conditions include GE/GT, or LE/LE operands. IMS does not support ranges.

While you can define non-unique search fields in DL/1 and ODBA maps, the read process ignores non-unique search fields when it searches for keys to be used for optimized reading. CCK columns can be reliably used while results are less predictable with other types of non-unique search fields.

For IMS randomized DEDB, HDAM, PHDAM database types, data is physically stored in a randomized sequence. If records must be fetched with unoptimized GN calls, the entire database is read.