利用DBMS_STATS包修改统计信息，欺骗优化器，生成糟糕的执行计划

在使用基于成本的优化器的优化器时，优化器生产执行计划时要估算每条SQL的执行成本，选择最佳的执行计划来执行sql语句。通过操纵统计信息就可以简介操纵执行计划的生成。
当然 需要强调的一点是，这是非常危险的行为

1 创建测试表

SQL> create table test_stats  as
2  select * from dba_objects ;
Table created.

2 收集统计信息

SQL> EXEC dbms_stats.gather_table_stats(ownname => 'DAO',tabname => 'TEST_STATS',cascade => TRUE);
PL/SQL procedure successfully completed.

3 创建索引

SQL> alter table TEST_STATS
2    add constraint dao_stats_pk primary key (OBJECT_ID);
Table altered.

4 统计表行数并查看执行计划

SQL> select  count(*)
2  from dao.test_stats  t ;
COUNT(*)
----------
75675
Execution Plan
----------------------------------------------------------
Plan hash value: 2395052010
------------------------------------------------------------------------------
| Id  | Operation             | Name         | Rows  | Cost (%CPU)| Time     |
------------------------------------------------------------------------------
|   0 | SELECT STATEMENT      |              |     1 |    44   (0)| 00:00:01 |
|   1 |  SORT AGGREGATE       |              |     1 |            |          |
|   2 |   INDEX FAST FULL SCAN| DAO_STATS_PK | 75675 |    44   (0)| 00:00:01 |
------------------------------------------------------------------------------
Statistics
----------------------------------------------------------
1  recursive calls
0  db block gets
164  consistent gets
0  physical reads
0  redo size
528  bytes sent via SQL*Net to client
523  bytes received via SQL*Net from client
2  SQL*Net roundtrips to/from client
0  sorts (memory)
0  sorts (disk)
1  rows processed

5 利用dbms_stats.set_table_stats 修改表统计信息

SQL> BEGIN
2  dbms_stats.set_table_stats(ownname =>'DAO',tabname => 'TEST_STATS',numrows => 1,numblks => 1);
3  END ;
4  /
PL/SQL procedure successfully completed.

SQL> select  t.owner,t.table_name,t.num_rows,t.blocks
2  from dba_tables t
3  where t.table_name ='TEST_STATS'
4  AND T.OWNER='DAO';
OWNER                          TABLE_NAME                       NUM_ROWS     BLOCKS
------------------------------ ------------------------------ ---------- ----------
DAO                            TEST_STATS                              1          1

6 再次查看统计信息

SQL> select  count(*)
2  from dao.test_stats  t ;
COUNT(*)
----------
75675
Execution Plan
----------------------------------------------------------
Plan hash value: 1525674154
-------------------------------------------------------------------------
| Id  | Operation          | Name       | Rows  | Cost (%CPU)| Time     |
-------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |            |     1 |     2   (0)| 00:00:01 |
|   1 |  SORT AGGREGATE    |            |     1 |            |          |
|   2 |   TABLE ACCESS FULL| TEST_STATS |     1 |     2   (0)| 00:00:01 |
-------------------------------------------------------------------------
Statistics
----------------------------------------------------------
0  recursive calls
0  db block gets
164  consistent gets
0  physical reads
0  redo size
528  bytes sent via SQL*Net to client
523  bytes received via SQL*Net from client
2  SQL*Net roundtrips to/from client
0  sorts (memory)
0  sorts (disk)
1  rows processed

本文出自 “道行尚浅（老道）的空间” 博客，请务必保留此出处http://daoye.blog.51cto.com/4190423/1431949