System Statistics and the CBO

I am just now getting around to implementing system statistics in my 9.2.0.6 
database that runs on an IBM P650 under AIX 5L. 

I have found a handful of explain plan differences with and without system 
statistics and am trying to understand why the plan changes when I import the 
system statistics.  Can someone explain to my why, once I import my system 
statistics, the CBO chooses a index that has a higher cardinality 
(X01CLAIM_FORM instead of X04CLAIM_FORM) for the following query:

System Stats:

PNAME                               PVAL1
------------------------------ ----------
CPUSPEED                              508
MAXTHR                           14344192
MBRC                                    4
MREADTIM                             .672
SLAVETHR                               -1
SREADTIM                            3.244

Without system stats:

Execution Plan
----------------------------------------------------------
   0      SELECT STATEMENT Optimizer=CHOOSE (Cost=3 Card=1 Bytes=97)
   1    0   TABLE ACCESS (BY INDEX ROWID) OF 'CLM_DETAIL' (Cost=1 Card=1 
Bytes=28)
   2    1     NESTED LOOPS (Cost=3 Card=1 Bytes=97)
   3    2       NESTED LOOPS (Cost=2 Card=1 Bytes=69)
   4    3         TABLE ACCESS (BY INDEX ROWID) OF 'CLAIM_FORM' (Cost=1 Card=1 
Bytes=39)

   5    4           INDEX (RANGE SCAN) OF 'X04CLAIM_FORM' (NON-UNIQUE) (Cost=2 
Card=1)

   6    3         TABLE ACCESS (BY INDEX ROWID) OF 'CLAIM' (Cost=1 Card=1 
Bytes=30)
   7    6           INDEX (RANGE SCAN) OF 'XPKCLAIM' (UNIQUE) (Cost=1 Card=1)
   8    2       INDEX (RANGE SCAN) OF 'XPKCLM_DETAIL' (UNIQUE) (Cost=1 Card=1)

With system stats:

Execution Plan
----------------------------------------------------------
   0      SELECT STATEMENT Optimizer=CHOOSE (Cost=4 Card=1 Bytes=97)
   1    0   TABLE ACCESS (BY INDEX ROWID) OF 'CLM_DETAIL' (Cost=2 Card=1 
Bytes=28)
   2    1     NESTED LOOPS (Cost=4 Card=1 Bytes=97)
   3    2       NESTED LOOPS (Cost=3 Card=1 Bytes=69)
   4    3         TABLE ACCESS (BY INDEX ROWID) OF 'CLAIM_FORM' (Cost=2 Card=1 
Bytes=39)

   5    4           INDEX (RANGE SCAN) OF 'X01CLAIM_FORM' (NON-UNIQUE) (Cost=2 
Card=153)

   6    3         TABLE ACCESS (BY INDEX ROWID) OF 'CLAIM' (Cost=2 Card=1 
Bytes=30)
   7    6           INDEX (RANGE SCAN) OF 'XPKCLAIM' (UNIQUE) (Cost=2 Card=1)
   8    2       INDEX (RANGE SCAN) OF 'XPKCLM_DETAIL' (UNIQUE) (Cost=2 Card=1)

Thanks,

Patty


 





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