Dienstag, 16. Dezember 2008

Detuning soft parses

During our seminar Oracle 10g Performance analysis with Doug Burns the question arises how to force the optimizer to do a hard-parse all the teme, even when prepared statements and bind variables are used. The question was about a DWH env.

For 11g and some backports I found DBMS_SHARED_POOL.PURGE which just purges the cursor (of course, you have to know it first; but that's no problem for one who complain about bind peeking in 10g).
And just for the records there are some Notes
Note:457309.1 - How To Flush an Object out the Library Cache [SGA]
Note:751876.1 - DBMS_SHARED_POOL.PURGE Is Not Working On
and other blogs also: Flushing a single cursor

But my initial idea was tha classic art or de-tuning:
Sometimes we complain about situatiions, where the statement is literally the same, but the optimizer creates several childs by hard-parsing. So let's use this bahaviour this time: Just change one of the many related optimizer environments.
1og Optimizer Environment Views will be a good start, but I assume (not tested!) even a NLS_SORT might help for the start.
Just grab one environment parameter which provides a wide range of values with little to no affect. So first bitmap_merge_area_size came into my mind, but I'm not sure if it will have no side effects, at least in a DWH env.

At the end, the common disclaimer: I have not tested any of the methods described here, so be warned ;-)

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