Optimizer dynamic sampling issues

Question

hi gurus,
emp_cur had 100K rows, i deleted many.
experimenting the dynamic sampling feature...
SQL> select * from v$version;
BANNER
Oracle Database 11g Enterprise Edition Release 11.1.0.6.0 - Production
PL/SQL Release 11.1.0.6.0 - Production
CORE    11.1.0.6.0      Production
TNS for 32-bit Windows: Version 11.1.0.6.0 - Production
NLSRTL Version 11.1.0.6.0 - Production
SQL> select name,value from v$parameter where name like 'optimizer_dynamic%';
NAME
VALUE
optimizer_dynamic_sampling
2
SQL> select count(*) from emp_par;
  COUNT(*)
      4999
SQL> set autotrace traceonly explain
SQL> select * from emp_par;
Execution Plan
Plan hash value: 3159588169
| Id  | Operation         | Name    | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT  |         |   100K|   878K|    63   (2)| 00:00:01 |
|   1 |  TABLE ACCESS FULL| EMP_PAR |   100K|   878K|    63   (2)| 00:00:01 |
-----------------------------------------------------------------------------since dynamic sampling in enabled, i expected the rows value close to 4999.
i tried to force dynamic sampling with a hint
SQL> select /*+ dynamic_sampling(t 2) */ * from emp_par t;
Execution Plan
Plan hash value: 3159588169
| Id  | Operation         | Name    | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT  |         |   100K|   878K|    63   (2)| 00:00:01 |
|   1 |  TABLE ACCESS FULL| EMP_PAR |   100K|   878K|    63   (2)| 00:00:01 |
-----------------------------------------------------------------------------though, dynamic sampling is enabled, optimizer plan is still showing the row count as 100K.
am i missing something?
thanks,
charles
Edited by: user570138 on Feb 23, 2010 9:43 PM

Answer 1

user570138 wrote:
hi gurus,
SQL> select /*+ dynamic_sampling(t 2) */ * from emp_par t;
Execution Plan
Plan hash value: 3159588169
| Id  | Operation         | Name    | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT  |         |   100K|   878K|    63   (2)| 00:00:01 |
|   1 |  TABLE ACCESS FULL| EMP_PAR |   100K|   878K|    63   (2)| 00:00:01 |
-----------------------------------------------------------------------------though, dynamic sampling is enabled, optimizer plan is still showing the row count as 100K.
Check the 10053 trace - the hint would have made the optimizer take a dynamic sample, but it has then rejected the sample as irrelevant.
If you want to change your test, try this:
<ul>
Create your table with 100,000 rows
Create the stats
Insert another 10%
Do the massive delete
</ul>
Then see if you get a difference between hinted and unhinted query plans.
(NB See also: http://jonathanlewis.wordpress.com/2010/02/23/dynamic-sampling/ )
Regards
Jonathan Lewis
http://jonathanlewis.wordpress.com
http://www.jlcomp.demon.co.uk
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