Tales From A Lazy Fat DBA

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When and why the optimizer switched to RBO from CBO mode … Why I cannot see that in 10053 trace ??

Posted by FatDBA on August 28, 2021

Hi Guys,

Recently I was working on one performance issue where one critical SQL statement started consuming more time. After a quick check I saw a switch was happened to RBO mode from CBO, but wasn’t sure on when and why the optimizer mode was switched. And the expected answer to my quest is to generate the debug 10053 trace file to get some insight about the cost-based optimizer internal calculations and to check the cardinality, selectivity and draw a good parallel with the way cost of table, index or sort etc. may be calculated.

Usually the best way to work out what’s going on in this situation is to look at the optimizer debug trace event, 10053 trace file. I always prefer to generate optimizer traces in such situations when the mighty optimizer messed up things. Being a performance consultant, it had saved me so many times in the past, always a best bet for me.

But this time it was looking little different, I couldn’t see details about why optimizer switched the mode in the ‘Query‘ section of the trace. I was totally perplexed, I mean this was not the first time I was looking for that information in the trace file. Why it’s not there, what happened .. 😦

This was Oracle 19.3.0.0.0 database running on RHEL, I tried metalink and found one document specific to this issue and luckily this was happening all due to a known bug 31130156. The problem was later on solved after we applied the bug-fix patch and interpreted the reason of the mode switch (I will write another post about the core problem) …

Note: It can be very difficult to interpret the 10053 optimizer trace if you don’t have any prior experience on it. I recommend readers to check one great document written by Wolfgang Breitling with title ‘A Look Under The Hood Of CBO’.

Hope It Helped!
Prashant Dixit

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