在Oracle 11.2.0.1.0下dbms_stats.gather_table_stats收集直方图不准

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 Linux: Version 11.2.0.1.0 - Production

NLSRTL Version 11.2.0.1.0 - Production





SQL> --制造一些数据

SQL> drop table test purge;

SQL> create table test as select * from dba_objects;

SQL> update test set object_id=2;

SQL> update test set object_id=1 where rownum=1;

SQL> commit;

SQL> create index ind_t_object_id on test(object_id);

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

SQL> --看看数据的分布

SQL> select object_id,count(1) from test group by object_id;

 OBJECT_ID   COUNT(1)

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

         1          1

         2      72415



SQL> set autotrace traceonly

SQL> --应该是要走索引

SQL> select * from test where object_id = 1;

运行计划

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

Plan hash value: 1357081020

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

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

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

|   0 | SELECT STATEMENT  |      | 36208 |  3359K|   290   (1)| 00:00:04 |

|*  1 |  TABLE ACCESS FULL| TEST | 36208 |  3359K|   290   (1)| 00:00:04 |

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

Predicate Information (identified by operation id):

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

   1 - filter("OBJECT_ID"=1)

统计信息

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

          1  recursive calls

          0  db block gets

       1039  consistent gets

          0  physical reads

          0  redo size

       1191  bytes sent via SQL*Net to client

        338  bytes received via SQL*Net from client

          2  SQL*Net roundtrips to/from client

          0  sorts (memory)

          0  sorts (disk)

          1  rows processed





SQL> --应该是要走全表扫描

SQL> select * from test where object_id = 2;

已选择72415行。

运行计划

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

Plan hash value: 1357081020

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

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

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

|   0 | SELECT STATEMENT  |      | 36208 |  3359K|   290   (1)| 00:00:04 |

|*  1 |  TABLE ACCESS FULL| TEST | 36208 |  3359K|   290   (1)| 00:00:04 |

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

Predicate Information (identified by operation id):

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

   1 - filter("OBJECT_ID"=2)

统计信息

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

          1  recursive calls

          0  db block gets

       5799  consistent gets

          0  physical reads

          0  redo size

    2940934  bytes sent via SQL*Net to client

      53435  bytes received via SQL*Net from client

       4829  SQL*Net roundtrips to/from client

          0  sorts (memory)

          0  sorts (disk)

      72415  rows processed



SQL> set autotrace off

SQL> col TABLE_NAME format a10;

SQL> col COLUMN_NAME format a10;

SQL> col ENDPOINT_ACTUAL_VALUE  format a10;

SQL> col ENDPOINT_NUMBER  format 9999999;

SQL> col ENDPOINT_VALUE  format 999999;

SQL>--直方图有问题，又一次收集直方图

SQL> select * from user_tab_histograms s where s.table_name='TEST' and column_name='OBJECT_ID';

TABLE_NAME COLUMN_NAM ENDPOINT_NUMBER ENDPOINT_VALUE ENDPOINT_A

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

TEST       OBJECT_ID             0              1

TEST       OBJECT_ID             1              2

SQL> exec dbms_stats.gather_table_stats(user, 'test',cascade=>true, method_opt=>'for columns object_id size 2');



SQL> set autotrace traceonly

SQL> --还是不正确

SQL> select * from test where object_id = 1;

运行计划

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

Plan hash value: 1357081020

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

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

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

|   0 | SELECT STATEMENT  |      | 36208 |  3359K|   290   (1)| 00:00:04 |

|*  1 |  TABLE ACCESS FULL| TEST | 36208 |  3359K|   290   (1)| 00:00:04 |

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

Predicate Information (identified by operation id):

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

   1 - filter("OBJECT_ID"=1)

统计信息

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

          0  recursive calls

          0  db block gets

       1039  consistent gets

          0  physical reads

          0  redo size

       1191  bytes sent via SQL*Net to client

        338  bytes received via SQL*Net from client

          2  SQL*Net roundtrips to/from client

          0  sorts (memory)

          0  sorts (disk)

          1  rows processed





SQL> set autotrace off

SQL> select * from user_tab_histograms s where s.table_name='TEST' and column_name='OBJECT_ID';

TABLE_NAME COLUMN_NAM ENDPOINT_NUMBER ENDPOINT_VALUE ENDPOINT_A

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

TEST       OBJECT_ID             5391              2





SQL> --仅仅实用analyze收集直方图

SQL> analyze table test compute statistics for table for columns object_id size 2;



SQL> select * from user_tab_histograms s where s.table_name='TEST' and column_name='OBJECT_ID';

TABLE_NAME COLUMN_NAM ENDPOINT_NUMBER ENDPOINT_VALUE ENDPOINT_A

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

TEST       OBJECT_ID                1              1

TEST       OBJECT_ID            72416              2



SQL> set autotrace traceonly

SQL> select * from test where object_id = 1;

运行计划

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

Plan hash value: 255872589

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

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

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

|   0 | SELECT STATEMENT            |                 |     1 |    99 |     2   (0)| 00:00:01 |

|   1 |  TABLE ACCESS BY INDEX ROWID| TEST            |     1 |    99 |     2   (0)| 00:00:01 |

|*  2 |   INDEX RANGE SCAN          | IND_T_OBJECT_ID |     1 |       |     1   (0)| 00:00:01 |

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

Predicate Information (identified by operation id):

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

   2 - access("OBJECT_ID"=1)

统计信息

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

          1  recursive calls

          0  db block gets

          4  consistent gets

          0  physical reads

          0  redo size

       1191  bytes sent via SQL*Net to client

        338  bytes received via SQL*Net from client

          2  SQL*Net roundtrips to/from client

          0  sorts (memory)

          0  sorts (disk)

          1  rows processed





SQL> select * from test where object_id = 2;

运行计划

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

Plan hash value: 1357081020

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

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

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

|   0 | SELECT STATEMENT  |      | 72415 |  7001K|   290   (1)| 00:00:04 |

|*  1 |  TABLE ACCESS FULL| TEST | 72415 |  7001K|   290   (1)| 00:00:04 |

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

Predicate Information (identified by operation id):

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

   1 - filter("OBJECT_ID"=2)

统计信息

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

          1  recursive calls

          0  db block gets

       5799  consistent gets

          0  physical reads

          0  redo size

    2940934  bytes sent via SQL*Net to client

      53435  bytes received via SQL*Net from client

       4829  SQL*Net roundtrips to/from client

          0  sorts (memory)

          0  sorts (disk)

      72415  rows processed

SQL> set autotrace off