The sledge hammer approach is to select count(*) from t20 where c2 = 'N' and c3
= 'N';
and then generate your query with a cardinality hint.
You might want to hint the use of the index for the count(*) query.
Of course you already know this is the sort of thing that the CBO is supposed
to get right routinely and I believe you’ve already done everything correctly
to give it the best possible chance. Sigh.
Now I do take it from your one is Y, one is N values that this is actually an
either or in your database. IF I’m correct about that and C2=Y literally
implies that C3=N, they you might want to code it up that way, leaving C3 out
of the database entirely and out of queries as a predicate. If someone wants it
instantiated for them in a query, I supposed you could make C3 a virtual
column. I’m presuming a *lot* for that to be true, and the CBO should be
getting this right as you’ve done it. (C2 could actually be N for N and null
for Y if you really want to get the index as small as possible.)
mwf
From: oracle-l-bounce@xxxxxxxxxxxxx [mailto:oracle-l-bounce@xxxxxxxxxxxxx] On ;
Behalf Of Ls Cheng
Sent: Tuesday, July 31, 2018 12:03 PM
To: Oracle Mailinglist
Subject: extended statistics and non-existent combined values
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')
