and_equal:
这种方式需要查询条件里面包括所有索引列,然后取得每个索引中得到的rowid列表,然后对这些列表做merge join,过滤出相同的rowid后再去表中获取数据或者直接从索引中获得数据.
and_equal有一些限制,比如它只对单列索引有效,只对非唯一索引有效,使用到的索引不能超过5个,查询条件只能是”=”.在10g中,and_equal已经被废弃了,只能通过hint才能生效.
涉及索引满足and_equal的限制条件的情况
create table test as select * from dba_objects;
create index ind_test_owner on test(owner);
create index ind_test_object_name on test(object_name);
SELECT /*+ and_equal(test ind_test_owner ind_test_object_name)*/
OWNER, OBJECT_NAME
FROM TEST
WHERE OWNER = 'TEST'
AND OBJECT_NAME = 'TEST';
由于涉及到的索引都为单列索引,且所有索引都在查询中出现,谓词也都是使用=关系符号连接,所以我们可以看到add_equal索引生效.执行计划如下所示.
Execution Plan
———————————————————-
——————————————————————————-
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)|
——————————————————————————-
| 0 | SELECT STATEMENT | | 1 | 29 | 2 (0)|
|* 1 | AND-EQUAL | | | | |
|* 2 | INDEX RANGE SCAN| IND_TEST_OWNER | 1 | | 1 (0)|
|* 3 | INDEX RANGE SCAN| IND_TEST_OBJECT_NAME | 2 | | 1 (0)|
——————————————————————————-
如果查询条件只包含owner
SELECT /*+ and_equal(test ind_test_owner ind_test_object_name)*/
OWNER, OBJECT_NAME
FROM TEST
WHERE OWNER = 'TEST'
本例中只使用到了hint中的一个索引关联的列owner,所以导致and_equal提示无效.执行计划中只使用了一个索引.
Execution Plan
———————————————————-
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)|
———————————————————————————–
| 0 | SELECT STATEMENT | | 1 | 29 | 2 (0)|
| 1 | TABLE ACCESS BY INDEX ROWID| TEST | 1 | 29 | 2 (0)|
|* 2 | INDEX RANGE SCAN | IND_TEST_OWNER | 1 | | 1 (0)|
———————————————————————————–
查询条件是”>”的情况
SELECT /*+ and_equal(test ind_test_owner ind_test_object_name)*/
OWNER, OBJECT_NAME
FROM TEST
WHERE OWNER = 'TEST'
AND OBJECT_NAME > 'TEST';
本例中由于查询谓词中对列object_name使用了>符号,所以也会导致and_equal提示无效.
Execution Plan
———————————————————-
—————————————————————————————–
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)|
—————————————————————————————–
| 0 | SELECT STATEMENT | | 1 | 40 | 2 (0)|
|* 1 | TABLE ACCESS BY INDEX ROWID| TEST | 1 | 40 | 2 (0)|
|* 2 | INDEX RANGE SCAN | IND_TEST_OBJECT_NAME | 2 | | 1 (0)|
—————————————————————————————–
查询条件是in的情况
SELECT /*+ and_equal(test ind_test_owner ind_test_object_name)*/
OWNER, OBJECT_NAME
FROM TEST
WHERE OWNER IN('TEST','zhanglei')
AND OBJECT_NAME IN('TEST')
In的情况有点特殊,如果in中的可选值有两个,那么将会导致add_equal提示失效.但如果in后面的可选值只有一个,oracle会将in当作=号来处理,同样可以使用and_equal提示.
Execution Plan
———————————————————-
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)|
—————————————————————————————–
| 0 | SELECT STATEMENT | | 1 | 40 | 2 (0)|
|* 1 | TABLE ACCESS BY INDEX ROWID| TEST | 1 | 40 | 2 (0)|
|* 2 | INDEX RANGE SCAN | IND_TEST_OBJECT_NAME | 2 | | 1 (0)|
————————