Tuesday, November 17, 2009

Explain Plan vs Real Plan on Oracle

A good part of my presentation Friday at NoCOUG was on explain plan command being an estimate of the actual execution plan. Now on the Oracle-L forum list, I see the same question being asked "Under what conditions, could the plan I get out of ‘explain plan’ differ from what I get from running the sql statement?" and Tanel Poder giving a succinct clear list of reasons:
main reasons:
1) The optimizer statistics the EXPLAIN PLAN ends up using are different from the statistics the other session ended up using
2) Explain plan does not use bind variable peeking thus will not optimize for current bind variable values
3) Explain plan treats all bind variables as VARCHAR2, thus you ma have implicit datatype conversion happening during the plan execution, (meaning to_char,to_number functions are added around variables/columns) and this for example may make optimizer to ignore some indexes if you get unlucky.
with the caveat:
Little correction to point number 3 - of course explain plan doesn't really execute the plan so the implicit datatype conversion you see is in the explained plan only, but if you actually execute the statement (with correct bind datatypes) then there's no implicit datatype conversion. And that's where the difference comes from...
From the presentation Friday at NoCOUG:
more examples on previous blog entry

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