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Questions about plan stability

EXPERT RESPONSE FROM: Paul Baumgartel

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QUESTION POSED ON: 29 December 2005
Plan stability is a major concern for me right now. Frequently when I update stats, a plan(s) changes and I get into hot water. I'm investigating stored outlines, but I have two concerns and they are: (1) I have a lot of current plans that are terrible and I don't want to lock them in with the others. (2) It's my understanding that for every cursor parsed, using stored outlines, a table must be hit to determine if an outline exists for it. I'm worried about the overhead of this process.

EXPERT RESPONSE
I don't have much experience with stored outlines, but with respect to question 1, you can store outlines just for selected SQL via:

CREATE OUTLINE outline [FOR CATEGORY category] ON statement

To gauge the overhead of using stored outlines, set up a trace of SQL that uses them, and compare it to a trace of the same SQL without using stored outlines. Remember that the lookup of a stored outline may be less expensive than constructing a new optimizer plan.

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