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Don Burleson Blog 







Oracle 11g dbms_stats AUTO sample size tips

Oracle Database Tips by Donald BurlesonJanuary 23, 2015

See this link for a complete treatment of 11g enhancements to statistics collection.

Providing the Oracle cost-based optimizer with information about dynamic tables has always been a problem.  In large databases, analyzing 50% of the rows in a table could take many hours, and in a dynamic environment the changes to the table will make the dbms_stats metadata sub-optimal, no matter how often we re-analyze CBO statistics.  This was one reason for implementing the 11g new feature of dynamic sampling, but Oracle has improved dbms_stats again in Oracle 11g.

This article notes that 11g has improved the AUTO parameter in dbms_stats.

?A fixed sampling percentage size that was good at some point in time may not be appropriate after the data distribution in the table has changed.

On the other hand when AUTO value is used Oracle will adjust the sample size when the data distribution changes.?

An issue of skew

A one-sized-fits all approach to sample sizes has issues, especially when the sample size is too small to recognize a skewed distribution of values.  Taking too small a sample from a table with skewed column values can cause the SQL optimizer to generate sub-optimal execution plans:

?When AUTO is used Oracle picks a sample size where the statistics quality is good enough. However, it does not work very well under cases of extreme skew in the data.?

Choosing the right sample size is not limited to Oracle CBO statistics; it's an issue with sampling a statistically representative sample from any universe..  This issue has been widely studied by statisticians across America, and it's nice to see that Oracle has chosen to change the AUTO sample size to be more robust:

?In Oracle 11g, we improved the behavior when the AUTO value is used. First, AUTO sampling now generates deterministic statistics. Second, and more importantly, AUTO sampling generates statistics that are almost as accurate as 100% sampling but takes much less time than 100% sampling.?

Bottom line:  The Oracle 11g dbms_stats AUTO sample size runs 10 times faster than 100% sampling and yields the statistics that asymptotically approach a time-consuming 100% sample.

If you like Oracle tuning, you might enjoy my book "Oracle Tuning: The Definitive Reference", with 950 pages of tuning tips and scripts. 

You can buy it direct from the publisher for 30%-off and get instant access to the code depot of Oracle tuning scripts.



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