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sqldb2query-optimizationexplain

How do I use DB2 Explain?

How do I use DB2's Explain function? -- both to run it, and to use it to optimize queries. Is there a better tool available for DB2?

I've built queries before, but the only way I've had to tell how long they'd take is to run them and time them -- which is hardly ideal.

Edit: The answer for me turned out to be "You can't. You don't have and cannot get the access." Don't you love bureaucracy?

Solution

  • What you're looking for is covered by two Db2 utilities:

    1. The explain facility, which shows the optimizer's access plan and estimated resource cost for a specific query (based on current RUNSTATS statistics)
    2. 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 facility more than the design advisor, especially if I have the option of changing the underlying SQL of the statement that needs to be tuned. The db2expln command is a convenient way to run the explain facility from the command line for any SQL or XQuery statement. I commonly run db2expln multiple times when comparing the costs of different versions of a statement I'm tuning. It's important that your table and index statistics are up to date when running explain or the design advisor.