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Don Burleson Blog 








RULE hint still popular in Oracle 10g

For many years (since the release of Oracle8), Oracle has threatened to phase-out the rule-based optimizer (RBO).

For Oracle Applications, it was ironic that Oracle Apps used the RBO, and Oracle made a big deal out of announcing that Oracle Applications 11i eBusiness suite was moving to the cost-based SQL optimizer (CBO).

Of course, when you examine the actual SQL in v$sql, you see than hundreds of their SQL statements have the RULE hint!

The RULE hint is a trick used by many SQL tuning professionals, and it's the first thing that I try when tuning a SQL statement since it points to problems with optimizer statistics (see dbms_stats and CBO histograms).

We also see MOSC note 375386 where Oracle notes sub-optimal execution plans for RMAN in 10g release 2!  The solution?  Forget about fixing the CBO statistics, Oracle recommends this fix, added to your RMAN script:

"alter session set optimizer_mode=RULE";


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