Tuning of SQL queries requires understanding why DB2 chose a particular access path and determining whether or not that access path should perform as expected. The key to this is understanding what filter factors are, how they are used by the Optimizer and how to get DB2 to better estimate them (with more statistics, or by telling DB2 what the filter factor is).
The presentation was delivered at IDUG EMEA in Brussels this month. Enjoy!
Watch “The ABCs of Filter Factors (aka Selectivity)” on 
and YouTube 