one additional thought: For the extended expression I think you need 4 buckets.
Yes, two of them should be empty, but there are four cases and you need the
empty buckets, right?
mwf
From: oracle-l-bounce@xxxxxxxxxxxxx [mailto:oracle-l-bounce@xxxxxxxxxxxxx] On ;
Behalf Of Timur Akhmadeev
Sent: Tuesday, July 31, 2018 1:29 PM
To: exriscer@xxxxxxxxx
Cc: Oracle Mailinglist
Subject: Re: extended statistics and non-existent combined values
You need a fake histogram that would have N,N in there with cardinality 1.
On Tue, 31 Jul 2018 at 19:04, Ls Cheng <exriscer@xxxxxxxxx> wrote:
Hi
I have a problematic query which is not doing index scan.
I tried to fix the issue using extended statistics but without success. I have
following test case, the idea is the estimated cardinality combining column C2
and C3 should return as few rows as possible because the predicate C2 = N and
C3 = N does not return any rows but with extended statistics it is actually
estimating 915 rows. Anyone can think of an workaround?
C2 has only two distinct values, Y and N
C3 has only two distinct values, Y and N
C2 and C3 with both values N returns no rows.
Thanks
create table t20
(
c1 number,
c2 varchar2(5),
c3 varchar2(5)
);
insert into t20
select rownum,
case when object_type = 'TABLE' then 'N' else 'Y' end c2,
case when object_type = 'TABLE' then 'Y' else 'N' end c3
from dba_objects;
create index t20_i1 on t20(c2, c3);
select c2, c3, count(*) from t20 group by c2, c3;
C2 C3 COUNT(*)
----- ----- ----------
N Y 1994
Y N 71482
exec dbms_stats.gather_table_stats('LSC', 'T20', method_opt => 'FOR ALL COLUMNS
SIZE 1, FOR COLUMNS C2 SIZE 2, FOR COLUMNS C3 SIZE 2')
-- c2 = 'N' and c3 = 'N' returns cero rows
select * from t20 where c2 = 'N' and c3 = 'N';
no rows selected
Execution Plan
----------------------------------------------------------
Plan hash value: 287249393
--------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)|
Time |
--------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 2121 | 19089 | 11 (0)|
00:00:01 |
| 1 | TABLE ACCESS BY INDEX ROWID| T20 | 2121 | 19089 | 11 (0)|
00:00:01 |
|* 2 | INDEX RANGE SCAN | T20_I1 | 2121 | | 5 (0)|
00:00:01 |
--------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("C2"='N' AND "C3"='N')
-- create extended statistics for C2 and C3
select
dbms_stats.create_extended_stats('LSC', 'T20','(C2, C3)')
from dual;
exec dbms_stats.gather_table_stats('LSC', 'T20', method_opt => 'FOR ALL COLUMNS
SIZE 1, FOR COLUMNS C2 SIZE 2, FOR COLUMNS C3 SIZE 2 FOR COLUMNS (C2, C3) SIZE
2')
-- c2 = 'N' and c3 = 'N' returns cero rows but even with extended statistics it
estimates 915 rows
select * from t20 where c2 = 'N' and c3 = 'N';
no rows selected
Execution Plan
----------------------------------------------------------
Plan hash value: 287249393
--------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)|
Time |
--------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 915 | 8235 | 5 (0)|
00:00:01 |
| 1 | TABLE ACCESS BY INDEX ROWID| T20 | 915 | 8235 | 5 (0)|
00:00:01 |
|* 2 | INDEX RANGE SCAN | T20_I1 | 915 | | 2 (0)|
00:00:01 |
--------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("C2"='N' AND "C3"='N')
--
Regards
Timur Akhmadeev