14. Using the Query Optimizer : Optimizer Timeout
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Optimizer Timeout
If the query optimizer believes that the best plan it has found so far takes less time to execute than the time it has taken evaluating QEPs, then it times out. It stops searching for further QEPs and returns the best plan is has found up to that point.
To tell if the query optimizer timed out, you can check either of the following:
The header of the QEP for set qep diagrams. In QEP diagrams generated using set qep, the keywords "timed out" and "no timeout" are indicated in the QEP diagram header.
The QEP pane of the SQL Scratchpad window in VDBA. In the QEP pane, the Timeout check box is enabled if the optimizer timed out (and disabled if the optimizer did not time out).
Note:  The fact that the query optimizer has timed out on a particular query does not guarantee that a better QEP is found; it indicates that not all QEPs have been checked, and so potentially, a better QEP can be found.
Because it is elapsed CPU time that is being measured, QEPs that time out can change, depending on machine load.