Re: Better cardinality estimate when dialing optimizer_features_enable back

  • From: Randolf Geist <info@xxxxxxxxxxxxxxxxxxxxx>
  • To: greg@xxxxxxxxxxxxxxxxxx, Neil Kodner <nkodner@xxxxxxxxx>, Robert Freeman <robertgfreeman@xxxxxxxxx>
  • Date: Tue, 24 Nov 2009 23:47:12 +0100

> So.... looking through this thread.... any way that dynamic sampling 
> could be a bad boy here?
> 
> Just a wild, off the cuff thought....
> 
> RF
> 
> Robert G. Freeman
> Oracle ACE

That is actually very good idea  (all the time I had the impression I missed 
something obvious) - in principle dynamic sampling should be able to get the 
selectivity right.

In this particular case the question would be if the overhead induced by 
dynamic sampling would outweigh the benefit of it. It depends largely on the 
sample level required to get helpful estimates - it might be worth a try to add 
a "DYNAMIC_SAMPLING(LETTER_BIN 2)" hint - starting with a low sample level (may 
be even 1). Higher sampling levels will get the selectivity right but the 
overhead might be significant then.

Regards,
Randolf

Oracle related stuff blog:
http://oracle-randolf.blogspot.com/

Co-author of the forthcoming "OakTable Expert Oracle Practices" book:
http://www.apress.com/book/view/1430226684
http://www.amazon.com/Expert-Oracle-Practices-Database-Administration/dp/1430226684
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