Re: Better cardinality estimate when dialing optimizer_features_enable back
Date: Wed, 25 Nov 2009 13:39:26 +0100
Message-Id: <947716251_at_web.de>
> The value in user_tab_columns.density for letter_bin.prty_id is 0.
> 000430560539754607.
> Take a look at the screenshot http://bit.ly/6AwFE6
> The left-hand-side is OFE 10.2.0.4 and shows density used as 4.0482e-
> 06.
> Compare that with the ofe 10.1.0 version on the right and you'll see
> it uses 4.3056e-04, which is stored in the data dictionary.
>
> Having said that, I'm not sure what to set the density of the column
> to-I'd appreciate some help with this.
As I said previously - you should set it to the value already shown in the dictionary for DENSITY which would be in this case the 4.3056e-04. The other value 4.0482e-06 is the NewDensity that is not visible from the dictionary.
Setting it to the value should revert to the previous handling of density - picking up the one from the dictionary.
Of course you could set it to different values then - if it helps to come up with a reasonable estimate for most of your queries.
> Additionally, look at the highlighted Histogram section-HtBal vs
> Frequency - could that be a clue?
I think is is a side effect of the different optimizer code paths.
> And finally, I uploaded a few examples of using the dynamic_sampling
> hint using the simple-case query to
> http://www.neilkodner.com/dynamic_sampling_results.txt
>
> Looks like dynamic_sampling didn't kick in until level 6.
Possibly the sampling results are rejected with lower sampling levels - there are some sanity checks applied. This can be seen from the 10053 optimizer trace file.
It would be interesting to see the EXPLAIN PLAN results with more complex queries - in particular those that used an suboptimal plan so far.
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
Preisknaller: WEB.DE DSL Flatrate für nur 16,99 Euro/mtl.! http://produkte.web.de/go/02/
-- http://www.freelists.org/webpage/oracle-lReceived on Wed Nov 25 2009 - 06:39:26 CST