Re: extended statistics and non-existent combined values

Well, that depends on the version. In Oracle 12.2, no it will not. In Oracle 12.1, I believe that it depends on the value of optimizer_adaptive_features_enable parameter. I haven't tested the 11G version. You may be right about the 11G version.

On Thu, Aug 2, 2018 at 4:41 AM, Dominic Brooks <dombrooks_at_hotmail.com> wrote:

> Wouldn’t the optimiser then discard dynamic sampling results because there
> was no matching data in the sample?
>
> Sent from my iPhone
>
> On 1 Aug 2018, at 21:14, Mladen Gogala <gogala.mladen_at_gmail.com> wrote:
>
> A slightly lighter sledgehammer would be using /*+
> DYNAMIC_SAMPLING(table,11) */.
>
> On 7/31/2018 12:39 PM, Mark W. Farnham wrote:
>
> 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_at_freelists.org [mailto:oracle-l-bounce_at_
> freelists.org <oracle-l-bounce_at_freelists.org>] *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')
>
>
>
> --
> Mladen Gogala Database Consultant Tel: (347) 321-1217
>
>

-- 
Kindest regards,
Mladen Gogala
Sr. Oracle DBA

--
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