extended statistics and non-existent combined values

From: Ls Cheng <exriscer_at_gmail.com>
Date: Tue, 31 Jul 2018 18:03:26 +0200
Message-ID: <CAJ2-Qb_N-PzHRWS11dX1qrz_U9Fm8La8qc17-uDEYVJPQj0Xog_at_mail.gmail.com>

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')

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Received on Tue Jul 31 2018 - 18:03:26 CEST