Oracle11G多列收集统计信息实验

   在上一篇文章中解决了Oracle11g 多列收集统计信息--直方图的问题，现在来做一个实验验证，这是Oracle 11g的新特性。

SQL> select * from v$version;

BANNER

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Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - 64bit Production

PL/SQL Release 11.2.0.1.0 - Production

CORE    11.2.0.1.0      Production

TNS for 64-bit Windows: Version 11.2.0.1.0 - Production

NLSRTL Version 11.2.0.1.0 - Production

SQL> drop table test purge;

SQL> create table test as select * from dba_objects;

SQL> create index ind_tt_oo on test(owner,object_type);

SQL> exec dbms_stats.gather_table_stats(user,'test',cascade => true);

SQL> select owner, object_type, count(1)

      from test

     where owner = 'SYS'

       and object_type = 'CLUSTER'

     group by owner, object_type;

OWNER   OBJECT_TYPE      COUNT(1)

------- -------------- ----------

SYS     CLUSTER                10

--这种组合数据有10条

SQL> set autotrace traceonly

SQL> select * from test where owner='SYS' and object_type='CLUSTER';

执行计划

----------------------------------------------------------

Plan hash value: 3811756094

-----------------------------------------------------------------------------------------

| Id  | Operation                   | Name      | Rows  | Bytes | Cost (%CPU)| Time     |

-----------------------------------------------------------------------------------------

|   0 | SELECT STATEMENT            |           |   298 | 28906 |    15   (0)| 00:00:01 |

|   1 |  TABLE ACCESS BY INDEX ROWID| TEST      |   298 | 28906 |    15   (0)| 00:00:01 |

|*  2 |   INDEX RANGE SCAN          | IND_TT_OO |   298 |       |     2   (0)| 00:00:01 |--而评估出来是298条

-----------------------------------------------------------------------------------------

Predicate Information (identified by operation id):

---------------------------------------------------

   2 - access("OWNER"='SYS' AND "OBJECT_TYPE"='CLUSTER')

统计信息

----------------------------------------------------------

          1  recursive calls

          0  db block gets

          9  consistent gets

          0  physical reads

          0  redo size

       2450  bytes sent via SQL*Net to client

        519  bytes received via SQL*Net from client

          2  SQL*Net roundtrips to/from client

          0  sorts (memory)

          0  sorts (disk)

         10  rows processed

SQL> set autotrace off

SQL> declare

      v_name varchar2(30);

      begin

      v_name := dbms_stats.create_extended_stats(user,'test','(owner,object_type)');

    end;

    /

SQL> set autotrace traceonly

SQL> exec dbms_stats.gather_table_stats(user,'test',cascade => true,method_opt  => 'FOR ALL COLUMNS SIZE SKEWONLY FOR COLUMNS (owner,object_type)',no_invalidate=>FALSE);

SQL> select * from test where owner='SYS' and object_type='CLUSTER';

执行计划

----------------------------------------------------------

Plan hash value: 3811756094

-----------------------------------------------------------------------------------------

| Id  | Operation                   | Name      | Rows  | Bytes | Cost (%CPU)| Time     |

-----------------------------------------------------------------------------------------

|   0 | SELECT STATEMENT            |           |    13 |  1417 |     2   (0)| 00:00:01 |

|   1 |  TABLE ACCESS BY INDEX ROWID| TEST      |    13 |  1417 |     2   (0)| 00:00:01 |

|*  2 |   INDEX RANGE SCAN          | IND_TT_OO |    13 |       |     1   (0)| 00:00:01 |--评估出来是13条，基本接近

-----------------------------------------------------------------------------------------

Predicate Information (identified by operation id):

---------------------------------------------------

   2 - access("OWNER"='SYS' AND "OBJECT_TYPE"='CLUSTER')

统计信息

----------------------------------------------------------

          8  recursive calls

          0  db block gets

         11  consistent gets

          0  physical reads

          0  redo size

       2450  bytes sent via SQL*Net to client

        519  bytes received via SQL*Net from client

          2  SQL*Net roundtrips to/from client

          0  sorts (memory)

          0  sorts (disk)

         10  rows processed

总结：在Oracle优化SQL，很多时候是要让CBO评估准过滤数据的条数。