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RE: Very Strange Query Access Plan

From: Mercadante, Thomas F (LABOR) <>
Date: Thu, 4 Oct 2007 14:55:53 -0400
Message-ID: <ABB9D76E187C5146AB5683F5A07336FFE0902C@EXCNYSM0A1AJ.nysemail.nyenet>


I came to the same conclusion (logically) myself just today - that the sample being taken by dbms_stats (using AUTO_SAMPLE_SIZE) is probably the culprit in that it is not taking a decent sample. The bad DENSITY value calculated by dbms_stats was .007692308 when the query ran horrible. If the formula for calculating DENSITY is 1/NDV, then NDV for this value = 130. This all points to dbms_stats gathering a bad sample of data to calculate this number.

Thanks again to everyone who responded. I learned a hell-of-a-lot from this discussion!


-----Original Message-----
From: Greg Rahn [] Sent: Thursday, October 04, 2007 2:12 PM To: Mercadante, Thomas F (LABOR)
Cc: John Kanagaraj; Subject: Re: Very Strange Query Access Plan

Since you are on the default METHOD_OPT did not collect histograms. It wasn't until 10g that the default changed to collect them.

I think your data is a poster child for a problem that I have seen several times in recent years. This problem is with the sample of data that DBMS_STATS is gathering with. You don't mentioned what your ESTIMATE_PERCENT is (if you are using DBMS_STATS.AUTO_SAMPLE_SIZE or a fixed %). If the data has a significant skew, generally a high number of NDV with a very few values dominating the set of values, AUTO_SAMPLE_SIZE can have challenges getting a representative sample. This is due to the way AUTO_SAMPLE_SIZE works: that being an algorithm based on convergence of the values observed. With skewed data, often times the convergence happens before an representative NDV is calculated. In the case of a fixed %, the chosen value is too low to get a representative NDV count. I went into a real-world example last year at Oracle OpenWorld 2006 which is available here:

In your case you have >27% (5M of 18M) of the data having 2 values. That is pretty significant skew. In this case I would recommend finding a sample size that yields representative NDV counts or if you feel comfortable because you know your data well, setting the NDV manually.

The AUTO_SAMPLE_SIZE in 11g has been enhanced to eliminate the issue around NDV and skewed data. More on that here: ts/

Hope this helps.

On 10/4/07, Mercadante, Thomas F (LABOR) <> wrote:
> John,
> We were gathering stats with no "METHOD_OPT" option. And according to
> an Oracle SR, the calculation for density is *not* 1/NDV, but:
> DENSITY = SUM(1..NDV)(nocc^2)/(T^2)
> where T is the number of elements sampled, adjusted like nocc
> (i.e. values that span histogram buckets are removed).
> basically, for each distinct value (i.e. NDV) we count the number of
> occurences of that value (the nocc value) tossing any value that spans
> a histogram bucket."
> The SR Tech said that the simpler calculation "is a rough
> of the formula above."
> Funny thing - I tested several scenarios and the first calculation
> to hold.
> He suggested trying histograms with a various number of buckets and
> testing the result, taking a 10046 trace to see what is happening. If
> am not satisfied with my results, to submit a (possible) bug report.
> The skew of the data in this table is the real problem.
> 18,000,000 rows.
> Ssn column:
> 1,289,561 rows with a value of "undefined"
> 3,656,617 rows with a value of null
> 625,018 distinct values.
> So 4.8 million rows of bad data.
> Now, try and find a time to test this without killing my users!
> Tom


Greg Rahn

Received on Thu Oct 04 2007 - 13:55:53 CDT

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