There are many friends who can't take the courage to upgrade to 11gR2 because the potential features of 11gR2 may cause trouble for the stable operation of the system after upgrade. In fact, some features introduced by Oracle in the development of new RDBMS software have good ideas, but often these ideas will bring variables to the stable Application environment. The most obvious one is the execution plan when 10g/9i is upgraded to 11gR2.In addition, a series of features such as adaptive cursor sharing adaptive cursor, automatic serial direct path automatic judgment serial direct path read, deferred segment creation, GC read mostly DRM.... Have been proved to be unsuitable for a large number of domestic applications in a large number of cases.(What I want to do in this article is to give a list of 11gR2 optimizer features and other problematic features listed above that can be parameterized and optionally disabled, provided that your Applicaiton has been adequately tested for this feature and has no time or environment to test for these new featuresInstead of disabling the features, disabling the new features simply returns to the default behavior of the older version (typically 10gR2 10.2.0.4).You must ask, "If the 11gR2 feature is disabled, what else do I upgrade to?"The answer is: First of all, here is a list of features that disable 11gR2. If you are already familiar with some of these features, you can selectively disable them instead of all. If you are not familiar with them and can't test them, you are not afraid to introduce uncertainties to a stable system.Next, only the potentially "catastrophic" features that are enabled by default in the 11gR2 section are listed here. Other features, such as flash back archive and security efile, are not enabled by default and require you to manually open them to take effect without being affected by this list.( Note that to avoid abuse, I only list a portion of this list. If you do need it, go to the T.AskMaclean.com forum to download it and download the address delivery:      

REM ===============================FOR STABLE OPTIMIZER===================================

alter system set "_enable_automatic_sqltune"=false scope=both;       Automatic SQL Tuning Advisory enabled parameter  

#The following optimizer parameters can be set at session/system level, and general optimizer parameters can be modified online            
alter session set "_serial_direct_read"=false;
alter session set "_nlj_batching_enabled" = 0; 
alter session set "_optimizer_undo_cost_change" = '10.2.0.4'; -- 11.2.0.1
alter session set "_optimizer_null_aware_antijoin" = false; -- true
alter session set "_optimizer_extend_jppd_view_types" = false; -- true
alter session set "_replace_virtual_columns" = false; -- true
alter session set "_first_k_rows_dynamic_proration" = false; -- true
alter session set "_bloom_pruning_enabled" = false; -- true
alter session set "_optimizer_multi_level_push_pred" = false; -- true
alter session set "_optimizer_group_by_placement" = false; -- true
alter session set "_optimizer_extended_cursor_sharing_rel" = none; -- simple
alter session set "_optimizer_adaptive_cursor_sharing" = false; -- true
alter session set "_optimizer_improve_selectivity" = false ; -- true
alter session set "_optimizer_enable_density_improvements" = false; -- true
alter session set "_optimizer_native_full_outer_join" = off; -- force
alter session set "_optimizer_enable_extended_stats" = false; -- true
alter session set "_nlj_batching_enabled" = 0; -- 1
alter session set "_optimizer_extended_stats_usage_control" = 255; -- 224
alter session set "_bloom_folding_enabled" = false; -- true
alter session set "_optimizer_coalesce_subqueries" = false; -- true
alter session set "_optimizer_fast_pred_transitivity" = false; -- true
alter session set "_optimizer_fast_access_pred_analysis" = false; -- true
alter session set "_optimizer_unnest_disjunctive_subq" = false; -- true
alter session set "_optimizer_unnest_corr_set_subq" = false; -- true
alter session set "_optimizer_distinct_agg_transform" = false; -- true
alter session set "_aggregation_optimization_settings" = 32; -- 0
alter session set "_optimizer_connect_by_elim_dups" = false; -- true
alter session set "_optimizer_eliminate_filtering_join" = false; -- true
alter session set "_connect_by_use_union_all" = old_plan_mode; --true
alter session set "_optimizer_join_factorization" = false; -- true
alter session set "_optimizer_use_cbqt_star_transformation" = false; -- true
alter session set "_optimizer_table_expansion" = false ; -- true
alter session set "_and_pruning_enabled" = false ; -- true
alter session set "_optimizer_distinct_placement" = false ; -- true
alter session set "_optimizer_use_feedback" = false ; -- true
alter session set "_optimizer_try_st_before_jppd" = false ; -- true

REM ===============================MEMORY===================================

alter system set "_memory_imm_mode_without_autosga"=false scope=both; 
alter system set "_enable_shared_pool_durations"=false scope=spfile;

REM ===============================SEGMENT==================================
alter system set deferred_segment_creation=false; 

.................