GU is used to retrieve the first segment that satisfies the specified SSA and establishes a starting point for sequential processing. The search start point of GU is the beginning of the database (i.e., the root level). After locating the first segment that satisfies the call, the current position is the starting position for sequential processing.

GHU locks the segment for sequential write operation (e.g., replace, delete, etc.), in addition to GU. GHU is not applicable for GSAM.