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Don Burleson Blog 







Oracle Deprecates cursor_sharing=similar

Oracle Database Tips by Donald BurlesonApril 25, 2015

It's well known that the setting cursor_sharing=similar has caused issues for many years.  

Oracle has always had an issue with implementing cursor peeking, a method whereby Oracle re-evaluates an execution plan to consider the value of a reentrant host variable.

 To use peeking, BOTH of the following condition must be true:

  1.  You have reentrant SQL using host variables.
  2.  The referenced column is heavily skewed, such that value of the host variable would influence the optimizer choice of execution  plan.  

This is a very rare case, such as a host variable with a highly skewed value, such that one host variable would have the "best" explain plan with a full-table scan while a more unpopular host variable value would be "best" with an index scan.  

Starting with 11g, Oracle implemented a  more intelligent solution to this rare case with the adaptive cursor sharing method to handle these rare cases, making cursor_sharing=similar obsolete.  

Oracle has decided to deprecate cursor_sharing=similar, a smart move since the overhead of peeking into a bind variable is more expensive than the unnecessary invocation of a full-table scan for a super-popular bind variable value.

 The My Oracle Support Community (MOSC) as announced the depreciation of the cursor_sharing=similar setting, per MOSC Note 1169017.1 titled Deprecating the cursor_sharing = 'SIMILAR' setting:

"We recommend that customers discontinue setting cursor_sharing = SIMILAR due to the many problematic situations customers have experienced using it.  

The ability to set this will be removed in version 12 of the Oracle Database (the settings of EXACT and FORCE will remain available).  

Instead, we recommend the use of Adaptive Cursor Sharing in 11g."

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