What does it indicate to see a query that has a low cost in the explain plan but a high consistent gets count in autotrace? In this case the cost was in the 100’s and the CR’s were in the millions.
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What does it indicate to see a query that has a low cost in the explain plan but a high consistent gets count in autotrace? In this case the cost was in the 100’s and the CR’s were in the millions.
The cost can represent two different things depending on version and whether you are running in cpu-based costing mode or not.
Briefly, the cost represents that amount of time that the optimizer expects the query to execute for, but it is expressed in units of the amount of time that a single block read takes. For example if Oracle expects a single block read to take 1ms and the query to take 20ms, then the cost equals 20.
Consistent gets do not match exactly with this for a number of reasons: the cost includes non-consistent (current) gets (eg reading and writing temp data), the cost includes CPU time, and a consistent get can be a multiblock read instead of a single block read and hence have a different duration. Oracle can also get the estimate of the cost completely wrong and it could end up requiring a lot more or less consistent gets than the estimate suggested.
A useful method that can helo explain disconnects between predicted execution plan and actual performance is ‘cardinality feedback’. See this presentation: http://www.centrexcc.com/Tuning%20by%20Cardinality%20Feedback.ppt.pdf