Oracle 11g release 2 upgrade performance problems

Question:  I saved a heap of problem with your 10g upgrade gotchas and I wonder what perils await me as I prepare to migrate to Oracle 11g r2?

Answer:  There are several 11g release 2 upgrade gotchas, dealing with direct read issues.  Also see MOSC Note 1392633.1 titled "Things to Consider Before Upgrading to 11.2.0.3 to Avoid Poor Performance or Wrong Results".

First, MAKE SURE that you follow the directions and capture all of your 10g SQL plans.   Many shop will implement plan stablity when migrating to 11g, and only turn-off the frozen plans after complete testing.  Also run dupgdiag.sql to check the database integrity prior to migrating to 11g.

Please see MOSC Note 1313429.1 titled: "Is it required to re-install the UTL_* packages after an upgrade to 11.2 ?"

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For Oracle 11g performance problems, call BC at 800-766-1884.  We are expert at Oracle10g performance problems and we know many tricks that are not published by Oracle for achieving optimal tuning.

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Here are some commonly-reported issues with Oracle 11g performance:

• _optimizer_extend_jppd_view_types tips There are some bug involved in 11g r2, and are fix by setting this parameter to false.

• Wrong results using GROUP BY placement where wrong results can occur when using GROUP BY placement where the aggregate column gets pruned from select list.  This is caused when required aggregate columns are missing in the plan in the projection list of operations.   See
  _optimizer_group_by_placement tips
  Workaround is to set set "_optimizer_group_by_placement"=FALSE

• Direct read issues:  There are reports of excessive direct reads after a migration to 11g r2. See my Adaptive path read operation tips  Issue the following statement to workaround the direct read issue.

       alter system set events '10949 trace name context forever';

• Resource Manager enables itself: The Oracle 11g resource manager now turns-on by default. Check the value of the resource_manager_plan init.ora parameter. If it is set to DEFAULT_PLAN you may want to change it to a more appropriate value, or use the following command to disable the resource manager:
  
  alter system set resource_manager_plan='' scope both;

• Disable AMM: Automatic Memory Management has issues since its inception and by 11g release 2 it remains problematic, and in some cases ASMM should be disabled in 11g release 2.  See MOSC note 793845.1  titled: "High direct path read waits in 11g".  See this important AMM bug and the associated fix

• Optimizer enhancements: Oracle says that the lack of extended statistics is the #1 cause of poor plans in 11g. With each release of Oracle we see improvements to the SQL optimizer, and you can expect to see many plan improve, but some which may get worse. Unless you like surprises, it's an Oracle best practice to "freeze" SQL execution plans with SQL profiles or Stored Outlines, and gradually introduce your SQL to the new optimizer. If you want to revert to an older release of the SQL optimizer you can enter "alter session set optimizer_feature_enable=10.1.1."

• Beware of  bug 9842771. This bug leads to wrong values in AUX_STATS$ for sreadtim and mreadtim by factor 1,000 therefore guiding the optimizer sometimes into the totally wrong direction. The workaround is to overwrite these values manually and divide them by 10,000. To make this change, use the dbms_stats.set_system_stats procedure. See this MOSC Note: 9842771.8 for the above bug for some further information. This issue is fixed in Oracle Database 11.2.0.3 and above.

• SQL Plan Manager: The 10g SQL profile approach is deprecated, and the 11g SQL plan management uses only two parameters, optimizer_capture_sql_plan_baselines and optimizer_use_sql_plan_baselines.

• Optimizer statistics: For all indexes with functions (function-based indexes) make sure to analyze them with the new 11g extended optimizer statistics.

• Double check your SYSAUX tablespace status: If the SYSAUX tablespace is offline in 11g, this can cause significant performance impact because required needed SQL Profiles are not accessible.

• ORA-12853   in dbms_stats:  When moving to 11gr2, some users report this error from the incorrect computation of dbms_stats.default_degree:

DBMS_STATS.GATHER_TABLE_STATS( OWNNAME => 'owner_name', TABNAME=> 'table_name' ,METHOD_OPT=>'FOR ALL INDEXED COLUMNS', ESTIMATE_PERCENT => DBMS_STATS.AUTO_SAMPLE_SIZE, DEGREE => DBMS_STATS.DEFAULT_DEGREE, CASCADE => TRUE, NO_INVALIDATE => TRUE);

ORA-12801: error signaled in parallel query server P057, instance<instance_name> (2)

ORA-12853: insufficient memory for PX buffers: current 958640K, max needed 11666304K

ORA-04031: unable to allocate 65560 bytes of shared memory ("large pool","unknown object","large pool","PX msg pool")

ORA-06512: at "SYS.DBMS_STATS", line 23828

• Make sure to see these important notes on _optimizer_cost_based_transformation issues when doing 11g upgrades. Also note that 9i has many optimizer bugs, notably issues that are corrected with _optimizer_transitivity_retain when using query re-write.

_optimizer_cost_based_transformation = OFF

IN-List costing:  There are many reports of shops migrating to Oracle 11g who suddenly experience SQL slowdowns.  One of the parameters that Oracle technical support is recommending is to set _optimizer_better_inlist_costing=true.

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