How do I use DB2 Explain?
IBM offers Data Studio as a free tool built on eclipse, which among other benefits includes a GUI for running visual explain, as well as providing tuning help through a query adviser. I highly recommend using Data Studio.
It is relatively easy to set up the correct resources (the explain tables that need to be built, and the bind that need to be done) by right clicking a connected data source, and choosing
analyze and tune > configure for tuning > guided configuration.
To generate the explain graph - simply highlight your query, right click, and choose "Open Visual Explain":
To use the query advisor, choose "start tuning" instead. It will take you through a process which will generate the explain, as well as recommend any tuning opportunities it can determine.
What you're looking for is covered by two DB2 utilities:
- The explain utility, which shows the optimizer's access plan and estimated cost for a specific query (based on current RUNSTATS statistics)
- The design advisor, which recommends structural changes to improve the performance of one or more queries
Both utilities require specialized tables to be created in the database.
I tend to use the explain utility more than the advisor, especially if I am able to change the SQL for the query to influence a better access plan. I use the db2expln command-line utility to explain different versions of a query I'm tuning and compare their costs. What's most important is that your table and index statistics are up to date when running explain or the design advisor.