联合索引优化方式选择

联合索引的使用，如果在表的两个字段上建立联合索引，在查询时如果谓词中没有出现第一个字段，仍然可以通过index skip scan的方式实现索引访问的方式。但是要注意的是不是所有的情况使用index skip scan都是最优的。当联合索引的第一个字段相同的值很多，大部分是相同的，这种情况下使用index skip scan的方式是最优的。

SQL> create table t as select 1 id, object_name from dba_objects;

Table created.

SQL> insert into t select 2, object_name from dba_objects;

50319 rows created.

SQL> insert into t select 3, object_name from dba_objects;

50319 rows created.

SQL> insert into t select 4, object_name from dba_objects;

50319 rows created.

SQL> commit ;

Commit complete.

SQL> select id,count(*) from t group by id;

ID COUNT(*)

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

1 50319

2 50319

4 50319

3 50319

可以看到表t的值集中在ID为1,2,3,4几个值上面，这时选择Index Skip Scan的访问方式是最优的。分析如下：

SQL> select * from t where object_name='TEST';

no rows selected

Execution Plan

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

Plan hash value: 3688940926

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

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

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

| 0 | SELECT STATEMENT | | 7 | 189 | 6 (0)| 00:00:01 |

|* 1 | INDEX SKIP SCAN | IND_T | 7 | 189 | 6 (0)| 00:00:01 |

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

Predicate Information (identified by operation id):

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

1 - access("OBJECT_NAME"='TEST')

filter("OBJECT_NAME"='TEST')

Statistics

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

1 recursive calls

0 db block gets

14 consistent gets

13 physical reads

0 redo size

388 bytes sent via SQL*Net to client

458 bytes received via SQL*Net from client

1 SQL*Net roundtrips to/from client

0 sorts (memory)

0 sorts (disk)

0 rows processed

当使用hints使其采用全表扫描的方式时

SQL> select /*+ full(t) */ * from t where object_name='TEST';

no rows selected

Execution Plan

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

Plan hash value: 1601196873

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

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

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

| 0 | SELECT STATEMENT | | 7 | 189 | 203 (3)| 00:00:03 |

|* 1 | TABLE ACCESS FULL| T | 7 | 189 | 203 (3)| 00:00:03 |

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

Predicate Information (identified by operation id):

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

1 - filter("OBJECT_NAME"='TEST')

Statistics

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

1 recursive calls

0 db block gets

900 consistent gets

0 physical reads

0 redo size

388 bytes sent via SQL*Net to client

458 bytes received via SQL*Net from client

1 SQL*Net roundtrips to/from client

0 sorts (memory)

0 sorts (disk)

0 rows processed

由上面可以看到，使用Index Skip Scan时只扫描了14个数据块，而采用全表扫描则扫描了900个数据块。

但是两外一种情况确实截然相反的，当联合索引的第一个值重复很少时，使用全表扫描的效率却要高一些。