Re: Better cardinality estimate when dialing optimizer_features_enable back
Date: Sun, 29 Nov 2009 19:11:32 -0800 (PST)
After missing a week, I'm just too tired to go through all the responses to this thread, so apologies upfront if I'm repeating a question already posted-
What is the optimizer_dynamic_sampling set to?
No matter if updated stats are present or not, I keep hearing that little voice in my head saying, "all it takes is the DATABASE deciding a dynamic sample needs to be taken due to the total pressure on the objects involved, by the total of the code involved and all the great stats gathering in the world goes right out the window..." :)
I have a puzzle box of code like this and I had to prove to other folks that it was occurring, as I just happened to notice a pattern to what the packages were being executed when we had an update statement with a different execution plan than the same process for the same statement... I used this pattern to know when to trace the process that was affected, traced them all out and then showed in the trace file the "OPT_DYN" statement that occurred. When this combination occurred and only when this combination occurred, the update statement had a 70% chance of extensive long-ops and another 10% of the select for the cursor would estimate days to complete. It was other combinations of packages and procs on the tables that were kicking off the dynamic sampling that then affected the simple update statement at certain times on another table, using two other tables as the sources for the cursor. Something to think about...:)
"Go away before I replace you with a very small and efficient shell script..."
- On Wed, 11/25/09, Neil Kodner <nkodner_at_gmail.com> wrote:
From: Neil Kodner <nkodner_at_gmail.com> Subject: Re: Better cardinality estimate when dialing optimizer_features_enable back To: "Randolf Geist" <info_at_sqltools-plusplus.org> Cc: "Greg Rahn" <greg_at_structureddata.org>, "oracle-l-freelists" <oracle-l_at_freelists.org> Date: Wednesday, November 25, 2009, 12:12 PM
As far as things that might have changed statistics-wise, I'm not too sure. I can ask around, there are other DBAs in the org, but I dont think anyone would have changed the stats gathering methods. the gather_stats_job runs but it hasn't touched any of the tables in this specific schema.
One other thing that Randolf touched upon was the single block read time taking longer than multi-block. I calculated workload statistics again and received a similar response. Is this worth bringing up to the Sysadmins? I've since deleted the system stats and computed noworkload statistics instead.
On Wed, Nov 25, 2009 at 11:51 AM, Randolf Geist <info_at_sqltools-plusplus.org> wrote:
> Well look what I found while poking around sys.wri$_optstat_histhead_
> http://bit.ly/75oNst (screenshot)
> Could this have been a function of sample size changing? Could
> parallel have anything to do with it?
Since the density itself was so far not used for the cardinality estimate calculation this is merely an indicator that "something" has changed - the old density is calculated based on the histogram generated. So a significant change in the density might indicate that the underlying histogram has changed.
That was one of the questions I haven't raised yet - what could have changed, since I already had the impression that there was a time when the batch job performed better from your comments.
You can try to restore the old statistics from the statistics history to see if there were significant differences in the histogram generated and the sample size used to gather the statistics along with the number of distinct values gathered. This way you could also check what NewDensity got calculated with the old statistics and what the estimates looked like for your example queries.
The slowly increasing density suggests that the underlying data might slowly have changed to something now that might have changed the shape of the height-balanced histogram so that the density calculation was significantly affected.
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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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