Optimizer Dynamic Sampling doesn't work in our DB

Question

Hi,
I'm trying to explore Optimizer Dynamic Sampling functionality,but it seems that database use this feature only if I use /*+ optimizer_dynamic_sampling */ hint.
There is optimizer_dynamic_sampling=2 set on system level (init.ora) and I also set this parameter on session level. But dynamic sampling is fired only with hint. I would like to use this feature transparently without any hints needed.
Could it be some kind of bug or I'm doing something wrong?
Thanks. Filip
See example below.
DB version: Oracle Database 11g Enterprise Edition Release 11.2.0.3.0 - 64bit Production
OS version: AIX-Based Systems (64-bit)
/* Check parameter setting*/
select name,value,isdefault from v$parameter p
where p.NAME='optimizer_dynamic_sampling';
NAME VALUE ISDEFAULT
optimizer_dynamic_sampling 2 TRUE
/* Create table without STATS*/
create table test_sampling as select * from all_objects;
/* Create index */
create index ix_tstsam on test_sampling (object_name, owner);
/* Check if statistics exists*/
select table_name,num_rows,last_analyzed from all_tables a where a.table_name= 'TEST_SAMPLING';
TABLE_NAME NUM_ROWS LAST_ANALYZED
TEST_SAMPLING NULL NULL
/* Setting Dynamic sampling on session level* /
Alter session set optimizer_dynamic_sampling=2;
/************ Explain plan - Select without hint ************/
explain plan set statement_id='TST_NOHINT' for
select sa.object_name from test_sampling sa where sa.owner = 'X';
PLAN_TABLE_OUTPUT
Plan hash value: 2834243581
| Id | Operation | Name |
| 0 | SELECT STATEMENT | |
|* 1 | TABLE ACCESS FULL| TEST_SAMPLING |
Note
- rule based optimizer used (consider using cbo)
/************ Explain plan - Select WITH hint ************/
explain plan set statement_id='TST_HINT' for
select /*+ dynamic_sampling(2) */ sa.object_name from test_sampling sa where sa.owner = 'X';
PLAN_TABLE_OUTPUT
Plan hash value: 3916830885
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time
| 0 | SELECT STATEMENT | | 8 | 272 | 86 (2)| 00:00:02
|* 1 | INDEX FAST FULL SCAN| IX_TSTSAM | 8 | 272 | 86 (2)| 00:00:02
Note
- dynamic sampling used for this statement (level=2)

Answer 1

Yes, this was the cause. Optimizer mode was set to CHOOSE. If I change it to ALL_ROWS, Optimizer Dynamic Sampling works.
Thank you !