OK, I can follow that, but why the change between ABC% and AB% ?
Henry
-----Original Message-----
Wolfgang Breitling
Sent: Thursday, November 06, 2003 10:19 AM
To: Multiple recipients of list ORACLE-L
Actually, it has nothing to do with any of the table or index statistics.
OK, almost nothing. I suppose if Jonathan (Lewis) can get the optimizer to
do a FTS on an umpteen billion row table to retrieve a single row by its
prime key, one can concoct a scenario of statistics values, aided by init
or session parameters, that would cause the CBO to use a full table scan to
resolve "where name like 'AB%".
Back to the topic. I did a test and the situation is easily reproduceable.
What happens is that as the like comparison string gets short, the
selectivity of the predicate decreases ( if you look at the 10053 trace,
the TBSEL value increases but that is the same paradoxon as with
performance: if something gets faster, did its performance decrease? ) as
one would expect. The TBSEL selectivity value and the rate of its increase
depends on the length of the like comparison string and the average column
length. When it gets down to the transition from ABC% to AB%, that trend
breaks sharply and suddenly the selectivity increases by orders of
magnitude ( TBSEL decreases by a huge factor ). for "like A%" it decreases
again, but is still lower (depends on avg col length) than the selectivity
of "like ABC%".
You can see that in the following test. The cardinality reflects the
changes in the tbsel value (cardinality = tbsel * num_rows, which was
10,000 for the test).
select id from sam where name like 'ABCDEFGHI%';
card operation
----- ----------------------------------
1 SELECT STATEMENT
1 TABLE ACCESS BY INDEX ROWID SAM
1 INDEX RANGE SCAN SAM_IX
select id from sam where name like 'ABCDEFGH%';
card operation
----- -----------------------------------
1 SELECT STATEMENT
1 TABLE ACCESS BY INDEX ROWID SAM
1 INDEX RANGE SCAN SAM_IX
select id from sam where name like 'ABCDEFG%';
card operation
----- -----------------------------------
57 SELECT STATEMENT
57 TABLE ACCESS FULL SAM
select id from sam where name like 'ABCDEF%';
card operation
----- -----------------------------------
100 SELECT STATEMENT
100 TABLE ACCESS FULL SAM
select id from sam where name like 'ABCDE%';
card operation
----- -----------------------------------
178 SELECT STATEMENT
178 TABLE ACCESS FULL SAM
select id from sam where name like 'ABCD%';
card operation
----- -----------------------------------
317 SELECT STATEMENT
317 TABLE ACCESS FULL SAM
select id from sam where name like 'ABC%';
card operation
----- -----------------------------------
563 SELECT STATEMENT
563 TABLE ACCESS FULL SAM
select id from sam where name like 'AB%';
card operation
----- -----------------------------------
2 SELECT STATEMENT
2 TABLE ACCESS BY INDEX ROWID SAM
2 INDEX RANGE SCAN SAM_IX
select id from sam where name like 'A%';
card operation
----- -----------------------------------
297 SELECT STATEMENT
297 TABLE ACCESS FULL SAM
At 04:29 PM 11/5/2003, you wrote:
>Hi Goulet,
>
>The clustering factor on the index=37930
>number of distinct keys=38357
>number of leaf blocks=1075
Wolfgang Breitling
Oracle7, 8, 8i, 9i OCP DBA
Centrex Consulting Corporation
http://www.centrexcc.com
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