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Don Burleson Blog 







Oracle 11g changes to "order by" and "group by" SQL performance

Oracle Database Tips by Donald BurlesonAugust 18,  2015

Oracle improves the SQL optimizer (the cost-based optimizer, or "CBO") with every release, and this sometimes leads to unexpected changes in SQL execution speed.

Guy Harrison notes that recent changes (starting in 10g release 2) to the optimization of order by and group by SQL queries can cause sub-optimal execution plans under certain conditions.

"Starting with Oracle 10.2,  you may notice a significant degradation in relative performance when you combine a GROUP BY with an ORDER BY on the same columns. 

When a GROUP BY is associated with an ORDER BY on the same columns,  the Oracle optimizer may choose a SORT GROUP BY rather than the usually more efficient HASH GROUP BY. 

Using the SORT GROUP BY avoids adding a SORT ORDER BY to the plan, but the overall result is usually disappointing.

To get a better result, you can perform the GROUP BY in an in-line view and perform the ORDER BY in the outer query.  Use the NO_MERGE hint to prevent the two operations from being combined."



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