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Don Burleson Blog 








Oracle 11g multicolumn histograms will improve CBO

Oracle 11g Tips by Burleson Consulting



Milen Kulev, has noted a very important enhancement to the Oracle 11g SQL Cost-Based Optimizer (CBO), a new feature with many aliases:

- Hybrid histograms

- Multi-column histograms

- 11g extended statistics.

This 11g CBO enhancement could be the most important 11g new features of all, greatly improving the CBO (and therefore all SQ execution performance), by providing the CBO with better cardinality information.  Milen quotes from the 11g docs that Oracle 11g has new correlated CBO statistics in addition to multi-column histograms:

"better information to the CBO by correlating statistics, such as Number of Distinct Values (NDV) and histograms, on multiple column"

Mark Bobak has this great example of how these new 11g hybrid histograms might work to improve the 11g optimizer's cardinality estimates:

"If you have a column "BIRTH_MONTH" and a column "ZODIAC_SIGN", you can say that 1/12 of the population will have a BIRTH_MONTH of DECEMBER, and 1/12 of the population will have a ZODIAC_SIGN of TAURUS.


If you ask the optimizer to estimate how many people have a BIRTH_MONTH of DECEMBER and a ZODIAC_SIGN of TAURUS, it will estimate 1/144.


But, we know the answer is 0 (since TAURUS is only people born between Apr. 20th - May 20th).


With histograms on multiple columns, the optimizer will now be able to understand these types of correlations and handle them correctly.


This will allow for much better cardinality estimates."






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