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Don Burleson Blog 







Oracle 11g upgrade performance problems

Oracle Database Tips by Donald BurlesonMay 18, 2015

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

Answer:  There are several 11g upgrade gotchas, small surprises that can effect your database performance when you migrate to 11g.  First, MAKE SURE that you follow the directions and capture all of your 10g SQL plans.   Also run dupgdiag.sql to check the database integrity prior to migrating to 11g.

  • Cardinality feedback - This user determined an issue with cardinality feedback causing poor SQL performance and fixed it by setting "_optimizer_use_feedback" .

  • AMM Bug:  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."

  • 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.

  • Setting temp filesystem issues:  There is an issue with the tmpfs size, causing ORA-00845 error even though you have created a swap partition/file greater than your database memory_target (see AMM Bug). The default size of tmpfs on Linux (2.4 kernel and above) is approximately half the physical system memory. This limitation can be changed by editing the /etc/fstab file, removing the "defaults" attribute and specifying a maximum size for tmpfs.

  • 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:


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

  • 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='';

  • 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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