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Don Burleson Blog 







Using v$sys_optimizer_env and v$sess_optimizer_env 

Oracle Database Tips by Donald BurlesonJune 1,  2015

Oracle has several views v$sys_optimizer_env and v$sess_optimizer_env that allow you to see how a specific session has changed from the optimizer default values. 

Querying v$sys_optimizer_env will show you the default SQL cost based optimizer parameter values for your instance:

SQL> select

  2     name,

  3     value,

  4     isdefault

  5  from

  6  v$sys_optimizer_env

  7  order by 3,1;


NAME                                     VALUE                     ISD

---------------------------------------- ------------------------- ---

_pga_max_size                            419420 KB                 NO

active_instance_count                    1                         YES

bitmap_merge_area_size                   1048576                   YES

cpu_count                                8                         YES

cursor_sharing                           exact                     YES

hash_area_size                           131072                    YES

is_recur_flags                           0                         YES

optimizer_capture_sql_plan_baselines     false                     YES

optimizer_dynamic_sampling               2                         YES

optimizer_features_enable                        YES

optimizer_index_caching                  0                         YES 

optimizer_index_cost_adj                 100                       YES

optimizer_mode                           all_rows                  YES

optimizer_secure_view_merging            true                      YES

optimizer_use_invisible_indexes          false                     YES

optimizer_use_pending_statistics         false                     YES

optimizer_use_sql_plan_baselines         true                      YES

parallel_ddl_mode                        enabled                   YES

parallel_degree                          0                         YES

parallel_dml_mode                        disabled                  YES

parallel_execution_enabled               false                     YES

parallel_query_default_dop               0                         YES 

parallel_query_mode                      enabled                   YES

parallel_threads_per_cpu                 2                         YES

pga_aggregate_target                     2097152 KB                YES

query_rewrite_enabled                    true                      YES

query_rewrite_integrity                  enforced                  YES

result_cache_mode                        MANUAL                    YES

skip_unusable_indexes                    true                      YES

sort_area_retained_size                  0                         YES

sort_area_size                           65536                     YES

star_transformation_enabled              false                     YES

statistics_level                         typical                   YES

transaction_isolation_level              read_commited             YES

workarea_size_policy                     auto                      YES


Remember, these are silver bullet parameters, in the sense that a system-wide change to an optimizer parameter will change the entire landscape of SQL performance (and not always in a good way!)

However, there are cases where a change to a specific session can change the default values for a session:

     - The alter session command - alter session set optimizer_mode=first_rows;

     - An optimizer SQL hint - select /*+ first_rows */

     - Using pga_aggregate_target - You will have varying values for hash_area_size and sort_area_size

We can select a specific SID value from v$session and drop it into this query to see specifically what has changed (in this example the SID = 141;

SQL> select name, isdefault from v$ses_optimizer_env

  2  where sid = 141

  3  order by isdefault, name;


NAME                                     ISD

---------------------------------------- ---

_pga_max_size                            NO

active_instance_count                    YES

bitmap_merge_area_size                   YES

cpu_count                                YES

cursor_sharing                           YES

hash_area_size                           YES

is_recur_flags                           YES

optimizer_capture_sql_plan_baselines     YES

optimizer_dynamic_sampling               YES

optimizer_features_enable                YES

optimizer_index_caching                  YES


NAME                                     ISD

---------------------------------------- ---

optimizer_index_cost_adj                 YES

optimizer_mode                           YES

optimizer_secure_view_merging            YES

optimizer_use_invisible_indexes          YES

optimizer_use_pending_statistics         YES

optimizer_use_sql_plan_baselines         YES

parallel_ddl_mode                        YES

parallel_degree                          YES

parallel_dml_mode                        YES

parallel_execution_enabled               YES

parallel_query_default_dop               YES


NAME                                     ISD

---------------------------------------- ---

parallel_query_mode                      YES

parallel_threads_per_cpu                 YES

pga_aggregate_target                     YES

query_rewrite_enabled                    YES

query_rewrite_integrity                  YES

result_cache_mode                        YES

skip_unusable_indexes                    YES

sort_area_retained_size                  YES

sort_area_size                           YES

star_transformation_enabled              YES

statistics_level                         YES


NAME                                     ISD

---------------------------------------- ---

transaction_isolation_level              YES

workarea_size_policy                     YES


35 rows selected.


SQL> spool off




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