DUI: DB2 Under (your) Influence

摘要

At certain times it is advantageous to maneuver the DB2 Optimizer into selecting a specific "access path"... it can also be dangerous! This paper explores the various methods of inducing the Optimizer to take another path, including faking the catalog statistics, taking poetic license with predicates, SQL construction, etc., illustrating the good (and bad) results obtained. Also, DB2 UDB for OS/390 Version 6 has a new feature that allows users to give the Optimizer "hints", and the mechanics of that facility are also discussed here.