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Don Burleson Blog 







Oracle bind variable peeking tips

Oracle Database Tips by Donald BurlesonOctober 14,  2015

Bind variable peeking is used with SQL statements in the library cache to see if the value of a host variable is going to change the existing execution plan.  The overhead of bind variable peeking was significant, and it was enable by setting this paraneter:

alter system set cursor_sharing=similar scope=both;

These issues with bind variable peeking led to the 11g adaptive cursor sharing feature.

In order to make efficient use of the Oracle shared pool (library cache), SQL statements must be re-entrant, and a good way to do this is to replace literal values with bind variables:

select * from customer where name = 'Burleson";

select * from customer where name = ':var1";

You can use the cursor_sharing=force init.ora parameter to replace literals in SQL with bind variable; a Godsend for making SQ: re-entrant and relieving stress on the shared pool SGA region.  See my notes here:

Unfortunately, there is a downside to this because the "invisible" bind variable gives less information to the Oracle optimizer.  This is especially important then the bind variable value is skewed (e.g. has popular and unpopular values) because the optimizer might choose an inefficient index or an unnecessary full-scan.

Up until 11g 11g adaptive cursor sharing, the implementation of bind variable peeking could cause performance problems, and many shops would disable bind variable peeking by opening a SR on MOSC and getting permission to set _optim_peek_user_binds=false.

Note:  For testing purposes, you may want to turn-off optimizer bind variable peeking and this can be done in several ways::

  • Re-set the _optim_peek_user_binds=false hidden parameter.

  • Disable new histogram creation by re-analyze using dbms_stats with the argument:  method_opt=> 'for all columns size 1';



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