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Don Burleson Blog 









How does Oracle estimate table join cardinality?

Oracle Database Tips by Donald Burleson

In the absence of column histograms, Oracle CBO must be able to "guess" the result set size and he sometimes gets it wrong. This is one reason why the ORDERED hint is one of the most popular SQL tuning hints, because using the ORDERED hint allows you to specify that the tables be joined together in the same order that they appear in the FROM clause, like this:

select /+ ORDERED */
natural join
natural join
   professor = 'jones'
   course = 'anthropology 610';

Remember, if the values for the professor and course table columns are not skewed then it is unlikely that the 10g automatic statistics would have created histograms buckets in the dba_histograms view for these columns.

As we see, the Oracle CBO needs to be able to accurately estimate the final number of rows returned by each step of the query and then use schema metadata (from running dbms_stats) to choose the table join order that results in the least amount of "baggage" (intermediate rows) from each of the table join operations.

But this is a daunting task. When a SQL query has a complex WHERE clause it can if very difficult to estimate the size of the intermediate result sets, especially when the WHERE clause transform column values with mathematical functions.

Oracle9i introduced the new dynamic sampling method for gathering run-time schema statistics and it is now enabled by default in Oracle10g.

The above text is an excerpt from forthcoming book "Oracle Tuning: The Definitive Reference" by Donald K. Burleson.




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