RE: What are the gory details behind DBMS_STATS.AUTO_SAMPLE_SIZE?

Thanks. I assume you meant a 10046 Level 4 trace. I tried a level 12 trace (just because I always regret it when I don't), and it does expose some rather interesting pieces of code, just like Niall promised. I asked a Metalink engineer and haven't heard anything official, yet. Or even unofficial for that matter.

From: John Kanagaraj [mailto:john.kanagaraj_at_hds.com] Sent: Monday, June 05, 2006 5:16 PM
To: Schultz, Charles; oracle-l
Subject: RE: What are the gory details behind DBMS_STATS.AUTO_SAMPLE_SIZE? Charles,  

I have found some interesting stuff when I turned on a level 4 trace on a DBMS_STATS collection session using AUTO_SAMPLE_SIZE. Unfortunately, I did not document what I saw, but I do remember that it performed multiple scans.... Not a Good Thing (tm).  

Regards,
John Kanagaraj <><
DB Soft Inc
Phone: 408-970-7002 (W)   

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From: oracle-l-bounce_at_freelists.org
[mailto:oracle-l-bounce_at_freelists.org] On Behalf Of Schultz, Charles Sent: Friday, June 02, 2006 10:52 AM
To: oracle-l
Subject: What are the gory details behind DBMS_STATS.AUTO_SAMPLE_SIZE?

I am curious how the algorithms behind DBMS_STATS.AUTO_SAMPLE_SIZE actually pick a sample size (10.2.0.1). We have witnessed a case where the sample size for a particular column was reduced 91.6%, causing the number of distinct values to be inaccurate, further causing a query to choose an FTS rather than an index. As far as I can tell, the skew has not changed that much, if at all, and the overall volume has increased by ~1%.

I am aware of workarounds (compute stats, lock stats, use an outline, etc), but like I said, I am really curious why the algorithm made such a drastic jump in the first place. I could not easily locate an appropriate white paper on metalinks, and I get too many hits on Google. Can anyone help satiate my burning desire to know? =)

charles schultz
oracle dba
aits - adsd
university of illinois

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