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Oracle v$system_fix_control and v$session _fix_control tips

Oracle Database Tips by Donald BurlesonApril 26,  2015

The Oracle SQL optimizer has always been problematic (especially with histogram evaluation), and Oracle has introduced v$system_fix_control and v$session_fix_control  starting in 10gr2 to control whether CBO bug patches are enabled or disabled. 

Also see my notes on how to display optimizer features by release using v$system_fix_control.

Oracle 11g's data dictionary defines the v$system_fix_control view using the following source query:

select BUGNO, VALUE, SQL_FEATURE, DESCRIPTION, OPTIMIZER_FEATURE_ENABLE, EVENT, IS_DEFAULT from GV$SYSTEM_FIX_CONTROL where inst_id=USERENV('Instance')

For internal testing ONLY, we have the v$session_fix_control view that allows you to enable and disable specific optimizer features.  This SQL optimization fix control feature is enabled via a hidden parameter called _fix_control.  The _fix_control parameter should ONLY be adjusted with the knowledge and consent of Oracle technical support.

alter session set "_fix_control"='5483301:off';

Notes on v$system _fix_control are on MOSC.

  • MOSC Doc ID 5483301.8
  • Bug 6082745 and bug 5483301: Cardinality of 1 when predicate value non-existent in frequency histogram

select distinct
   bugno
from
   v$session_fix_control;

BUGNO
----------
3499674
4556762
4569940
3118776
4519016
4175830
4663698
4631959
4550003
4584065
4487253
4611850
4663804
4602374
4728348
4723244
4554846
4545833
4488689
4519340

20 rows selected.


 

 

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