有个徒弟问我，要创建一个索引，去优化一个SQL，可是创建了索引之后其它 SQL 也要用 这个索引，其它SQL慢死了。要优化的SQL又快。遇到这样的问题咋搞？

一般遇到这样的问题还是非常少的。处理的方法非常多。我简单的给大家介绍一种方法。

还是直接看我实验操作步骤吧。

在SCOTT账户里面创建一个測试表和一个索引
SQL> create table test as select * from dba_objects;

表已创建。
SQL> create index idx_test on test(object_id);

索引已创建。

SQL> set lines 200 pages 200
SQL> set autot trace
SQL> select * from test where object_id=10;


运行计划
----------------------------------------------------------
Plan hash value: 2473784974

----------------------------------------------------------------------------------------
| Id  | Operation                   | Name     | Rows  | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT            |          |     1 |   207 |     2   (0)| 00:00:01 |
|   1 |  TABLE ACCESS BY INDEX ROWID| TEST     |     1 |   207 |     2   (0)| 00:00:01 |
|*  2 |   INDEX RANGE SCAN          | IDX_TEST |     1 |       |     1   (0)| 00:00:01 |
----------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   2 - access("OBJECT_ID"=10)

Note
-----
   - dynamic sampling used for this statement (level=6)


统计信息
----------------------------------------------------------
         44  recursive calls
          0  db block gets
        136  consistent gets
          4  physical reads
          0  redo size
       1404  bytes sent via SQL*Net to client
        420  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed
然后人工设置索引统计信息 把集群银子搞大(很大)

SQL> begin
  2    dbms_stats.set_index_stats(ownname  => ‘SCOTT‘,
  3                               indname  => ‘IDX_TEST‘,
  4                               numrows  => 100000000000,
  5                               numlblks => 100000,
  6                               numdist  => 100000,
  7                               avglblk  => 100000,
  8                               avgdblk  => 100000,
  9                               clstfct  => 100000000000);
 10  end;
 11  /

PL/SQL 过程已成功完毕。

SQL> select * from test where object_id=10;


运行计划
----------------------------------------------------------
Plan hash value: 1357081020

--------------------------------------------------------------------------
| Id  | Operation         | Name | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |      |     1 |   207 |   290   (1)| 00:00:04 |
|*  1 |  TABLE ACCESS FULL| TEST |     1 |   207 |   290   (1)| 00:00:04 |
--------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - filter("OBJECT_ID"=10)

Note
-----
   - dynamic sampling used for this statement (level=6)


统计信息
----------------------------------------------------------
          0  recursive calls
          0  db block gets
       1039  consistent gets
          0  physical reads
          0  redo size
       1404  bytes sent via SQL*Net to client
        420  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed
这个时候。全部的SQL都不会走这个索引了，你想让某个SQL走索引。直接hint 让它走就ok了
SQL> select /*+ index(test idx_test) */ * from test where object_id=10;


运行计划
----------------------------------------------------------
Plan hash value: 2473784974

----------------------------------------------------------------------------------------
| Id  | Operation                   | Name     | Rows  | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT            |          |     1 |   207 |  1446K  (1)| 04:49:20 |
|   1 |  TABLE ACCESS BY INDEX ROWID| TEST     |     1 |   207 |  1446K  (1)| 04:49:20 |
|*  2 |   INDEX RANGE SCAN          | IDX_TEST |     1 |       |    15  (94)| 00:00:01 |
----------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   2 - access("OBJECT_ID"=10)

Note
-----
   - dynamic sampling used for this statement (level=6)


统计信息
----------------------------------------------------------
          0  recursive calls
          0  db block gets
          4  consistent gets
          0  physical reads
          0  redo size
       1404  bytes sent via SQL*Net to client
        420  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed


这样还没完，由于要是有人收集了统计信息，会覆盖我们set的统计信息，所以收集完统计信息之后，要再跑一下那个set的统计信息就ok了。