Optimal query access plans are essential for good data server performance and it is the Db2 query optimizer's job to choose the best access plan. The optimizer is a very sophisticated component of the data server, tasked with the challenging job of choosing good access plans for the variety of features and table organizations supported by Db2. The optimizer can automatically rewrite complex SQL resulting in huge performance improvements. It models various aspects of Db2 runtime so that it can choose the best access plan out of hundreds of thousands of possible options. Attend this session to get an overview of how the optimizer works and to get some tips on how to understand its decisions and control its behaviour.