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Don Burleson Blog 







Oracle Hash Aggregation Tips

Expert Oracle Database Tips by Donald BurlesonDecember 2, 2015

Question:  I am getting a slow GROUP BY performance in Oracle 10g and I believe that it is related to a hash aggregation step in my SQL execution plan. 

What is this new hash aggregation plan, and how do I fix this slow performance?

Answer:  The hash aggregation plan is an 10g enhancement that improves GROUP BY performance when enough PGA exists to allow hash operations to replace traditional sorting. 

Your slow GROUP BY performance is one of the 10g upgrade issues, and Oracle has released Bug 4604970 to address issues with the brand new hash aggregation plan in 10g.

Oracle suggests setting the hidden parameter _gby_hash_aggregation_enabled=false to disable hash aggregation. This bug should be fixed in Oracle 11g and beyond, and hash aggregation should improve performance of GROUP BY operations where enough RAM exists in the PGA to allow a hash operation.

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