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Don Burleson Blog 







Oracle ACE warns about optimizer bug

Oracle Database Tips by Donald BurlesonMay 29,  2015

Oracle ACE warns about optimizer bug

Randolph Geist, and Oracle ACE has identified an important bug on the Oracle SQL optimizer.  When using multiple blocksizes, the SQL optimizer may chose a sub-optimal execution plan:

"It really looks like that this feature has been introduced to support transportable tablespaces but it obviously hasn't been tested very thoroughly when it comes to cost calculation.

Each of the different modes has its deficiencies when dealing with objects in non-default blocksizes.

The somehow odd thing is that the traditional I/O costing does the best job, and all system statistics based calculations are utterly wrong."

The use of the Oracle multiple blocksize feature is not for everyone, and it's not useful in over 90% of all databases.  However, multiple blocksizes are used in TPC benchmark tests and they are useful for very large, highly active systems, deploying multiple blocksizes can make a huge difference in response time for certain queries. 

Oracle ACE and OCM Steve Karam, also notes unexplainable issues with multiple blocksizes, citing huge performance differences:

"The final result was a 270 times improvement over the original."

Geist does not offer any workarounds for this bug, which he attributes to software that "obviously hasn't been tested very thoroughly".

He also notes that using a different blocksize reduces logical I/O (consistent gets), an internal operation that sets locks and latches to ensure data concurrency and enforces read consistency, but claims that a 50% reduction in logical I/O may not make a statistically significant difference in performance.



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