Clustering Factor过高导致查询没走索引

昨天遇到一个sql，查询一直没走索引，而走的全表扫描，检查是因为群集因子太高导致不走索引，表的行数（NUM_ROWS=135,352,160），（Clustering Factor=110,160,620）

解决方法：把索引相对的列进行排序存储，减少群集因子

重建索引是不能改变群集因子大小的

集群因子反映了数据相对于索引的列是否有序

集群因子的计算： 

(1) 按顺序扫描索引。 

(2) 将当前索引值指向的ROWID的块部分与以前的索引值进行比较 

(3) 如果ROWID指向不同的TABLE块，则增加集群因子

模拟环境：

SQL> conn scott/tiger

Connected.

SQL> create table t1 as select * from emp;

SQL> select count(*) from t1;

  COUNT(*)

----------

   1835008

SQL> create index ind_t1_empno on t1(empno);  

SQL> analyze table t1 compute statistics;  --分析表

SQL> analyze index ind_t1_empno compute statistics; --分析索引

SQL> select index_name,CLUSTERING_FACTOR from user_indexes where index_name='IND_T1_EMPNO';  查看群集因子

INDEX_NAME                     CLUSTERING_FACTOR

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

IND_T1_EMPNO                              150850

SQL> set autot trace

SQL> select * from t1 where empno=7900;

131072 rows selected.

Elapsed: 00:00:02.04

Execution Plan

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

Plan hash value: 3617692013

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

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

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

|   0 | SELECT STATEMENT  |      |   131K|  4096K|  2501   (3)| 00:00:31 |

|*  1 |  TABLE ACCESS FULL| T1   |   131K|  4096K|  2501   (3)| 00:00:31 |

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

Predicate Information (identified by operation id):

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

   1 - filter("EMPNO"=7900)

Statistics

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

          0  recursive calls

          0  db block gets

      19587  consistent gets

       10878  physical reads

          0  redo size

    1765934  bytes sent via SQL*Net to client

      96518  bytes received via SQL*Net from client

       8740  SQL*Net roundtrips to/from client

          0  sorts (memory)

          0  sorts (disk)

     131072  rows processed

以上查询走了全表扫描，而没有走索引，因为群集因子太高

SQL> alter index ind_t1_empno rebuild; --重建索引

Index altered.

SQL> analyze index ind_t1_empno compute statistics;

Index analyzed.

SQL> select index_name,CLUSTERING_FACTOR from user_indexes where index_name='IND_T1_EMPNO';

INDEX_NAME                     CLUSTERING_FACTOR

------------------------------ -----------------
IND_T1_EMPNO                              150850

重建完索引，但是群集因子并没有减少

SQL> select * from t1 where empno=7900;

131072 rows selected.

Elapsed: 00:00:06.04

Execution Plan

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

Plan hash value: 3617692013

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

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

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

|   0 | SELECT STATEMENT  |      |   131K|  4096K|  2501   (3)| 00:00:31 |

|*  1 |  TABLE ACCESS FULL| T1   |   131K|  4096K|  2501   (3)| 00:00:31 |

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

Predicate Information (identified by operation id):

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

   1 - filter("EMPNO"=7900)

Statistics

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

       1061  recursive calls

          0  db block gets

      19743  consistent gets

      10900  physical reads

          0  redo size

    1765934  bytes sent via SQL*Net to client

      96518  bytes received via SQL*Net from client

       8740  SQL*Net roundtrips to/from client

         10  sorts (memory)

          0  sorts (disk)

     131072  rows processed

还是走全表扫描

SQL> create table t2 as select * from t1 order by empno; 

Table created.

SQL> create index ind_t2_empno on t2(empno);

Index reated.

SQL> analyze table t2 compute statistics;

Table analyzed.

SQL> analyze index ind_t2_empno compute statistics;

Index analyzed.

SQL> select index_name,CLUSTERING_FACTOR from user_indexes where index_name='IND_T2_EMPNO';

INDEX_NAME                     CLUSTERING_FACTOR

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

IND_T2_EMPNO                               10820

数据有序存储，减少了群集因子值

SQL> conn / as sysdba

Connected.

SQL> alter system flush buffer_cache;

System altered.

SQL> alter system flush shared_pool; 

System altered.

SQL> conn scott/tiger

Connected.

SQL> set autot trace

SQL> select * from t2 where empno=7900;

131072 rows selected.

Elapsed: 00:00:01.88

Execution Plan

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

Plan hash value: 632553325

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

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

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

| Time     |

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

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

|   0 | SELECT STATEMENT            |              |   131K|  4096K|  1058   (1)

| 00:00:13 |

|   1 |  TABLE ACCESS BY INDEX ROWID| T2           |   131K|  4096K|  1058   (1)

| 00:00:13 |

|*  2 |   INDEX RANGE SCAN          | IND_T2_EMPNO |   131K|       |   279   (2)

| 00:00:04 |

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

Predicate Information (identified by operation id):

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

   2 - access("EMPNO"=7900)

Statistics

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

       1061  recursive calls

          0  db block gets

      18605  consistent gets

       1006  physical reads

          0  redo size

    1765934  bytes sent via SQL*Net to client

      96518  bytes received via SQL*Net from client

       8740  SQL*Net roundtrips to/from client

         10  sorts (memory)

          0  sorts (disk)

     131072  rows processed