and_equal,index_join,index_combine

and_equal,index_join,index_combine这三种都是oracle利用索引关联获得数据的方法,三者的目的都是为了最大限度的利用索引,减少回表的代价.但是三者的实现方法是有区别的,下面一一来分析.

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.

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);

SQL 10G>select/*+ and_equal(test ind_test_owner ind_test_object_name)*/ owner,object_name from test where owner='test' and object_name='test';

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

SQL 10G>select/*+ and_equal(test ind_test_owner ind_test_object_name)*/ owner,object_name from test where owner='test';

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)|
-----------------------------------------------------------------------------------

修改owner,object_name为非空

alter table test modify(owner not null);
alter table test modify(object_name not null);

SQL 10G>select/*+ and_equal(test ind_test_owner ind_test_object_name)*/ owner,object_name from test where owner='test';

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)|
-----------------------------------------------------------------------------------

效果一样

查询条件是">"的情况

SQL 10G>select/*+ and_equal(test ind_test_owner ind_test_object_name)*/ owner,object_name,object_type from test where owner>'test' and object_name='test';

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
SQL 10G>select/*+ and_equal(test ind_test_owner ind_test_object_name)*/ owner,object_name,object_type from test where owner in('test','dba') and object_name='test';

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)|
-----------------------------------------------------------------------------------------

再来看一下回表的情况

SQL 10G>select/*+ and_equal(test ind_test_owner ind_test_object_name)*/ owner,object_name,object_type from test where owner='test' and object_name='test';

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 |   AND-EQUAL                 |                      |       |       |            |
|*  3 |    INDEX RANGE SCAN         | IND_TEST_OWNER       |     1 |       |     1   (0)|
|*  4 |    INDEX RANGE SCAN         | IND_TEST_OBJECT_NAME |     2 |       |     1   (0)|
-----------------------------------------------------------------------------------------

先通过and_equal取得rowid列表,然后从表中返回数据.

 

index_join:
index join顾名思义是对index进行关联,oracle通过hash index join的方式实现了避免对表的访问.所有的数据都从索引中直接获得.它不受查询条件影响,可以是唯一索引,也可以是多列索引.

SQL 10G>select/*+ index_join(test ind_test_owner ind_test_object_name)*/ owner,object_name from test where owner='test' and object_name='test';

Execution Plan
----------------------------------------------------------

--------------------------------------------------------------------------------
| Id  | Operation          | Name                 | Rows  | Bytes | Cost (%CPU)|
--------------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |                      |     1 |    29 |     3  (34)|
|*  1 |  VIEW              | index$_join$_001     |     1 |    29 |     3  (34)|
|*  2 |   HASH JOIN        |                      |       |       |            |
|*  3 |    INDEX RANGE SCAN| IND_TEST_OWNER       |     1 |    29 |     1   (0)|
|*  4 |    INDEX RANGE SCAN| IND_TEST_OBJECT_NAME |     1 |    29 |     1   (0)|
--------------------------------------------------------------------------------

可以不带查询条件,只不过由index range scan变成了index fast full scan

SQL 10G>select/*+ index_join(test ind_test_owner ind_test_object_name)*/ owner,object_name from test
  2  ;

Execution Plan
----------------------------------------------------------

------------------------------------------------------------------------------------
| Id  | Operation              | Name                 | Rows  | Bytes | Cost (%CPU)|
------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT       |                      | 51984 |  1472K|   478   (2)|
|   1 |  VIEW                  | index$_join$_001     | 51984 |  1472K|   478   (2)|
|*  2 |   HASH JOIN            |                      |       |       |            |
|   3 |    INDEX FAST FULL SCAN| IND_TEST_OWNER       | 51984 |  1472K|   153   (2)|
|   4 |    INDEX FAST FULL SCAN| IND_TEST_OBJECT_NAME | 51984 |  1472K|   322   (1)|
------------------------------------------------------------------------------------

如果不是所有数据都能从索引获得,那么将不会使用index join

SQL 10G>select/*+ index_join(test ind_test_owner ind_test_object_name)*/ owner,object_name,object_type from test where owner='test' and object_name='test';

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_OWNER |     1 |       |     1   (0)|
-----------------------------------------------------------------------------------

index_combine:
index combine最早是用在bitmap index上的,在9i开始oracle默认可以使用在btree索引上,这是由_b_tree_bitmap_plans参数来控制的.oracle将btree索引中获得的rowid信息通过BITMAP CONVERSION FROM ROWIDS的步骤转换成bitmap进行匹配,然后匹配完成后通过BITMAP CONVERSION TO ROWIDS再转换出rowid获得数据或者回表获得数据.

 

SQL 10G>select/*+ index_combine(test ind_test_owner ind_test_object_name)*/ owner,object_name from test where owner='test' and object_name='test';

Execution Plan
----------------------------------------------------------

---------------------------------------------------------------------------------------------
| Id  | Operation                       | Name                 | Rows  | Bytes | Cost (%CPU)|
---------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT                |                      |     1 |    29 |     2   (0)|
|   1 |  BITMAP CONVERSION TO ROWIDS    |                      |     1 |    29 |     2   (0)|
|   2 |   BITMAP AND                    |                      |       |       |            |
|   3 |    BITMAP CONVERSION FROM ROWIDS|                      |       |       |            |
|*  4 |     INDEX RANGE SCAN            | IND_TEST_OWNER       |       |       |     1   (0)|
|   5 |    BITMAP CONVERSION FROM ROWIDS|                      |       |       |            |
|*  6 |     INDEX RANGE SCAN            | IND_TEST_OBJECT_NAME |       |       |     1   (0)|
---------------------------------------------------------------------------------------------

回表取数据的情况

SQL 10G>select/*+ index_combine(test ind_test_owner ind_test_object_name)*/ owner,object_name,object_type from test where owner='test' and object_name='test';

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 |   BITMAP CONVERSION TO ROWIDS    |                      |       |       |            |
|   3 |    BITMAP AND                    |                      |       |       |            |
|   4 |     BITMAP CONVERSION FROM ROWIDS|                      |       |       |            |
|*  5 |      INDEX RANGE SCAN            | IND_TEST_OWNER       |       |       |     1   (0)|
|   6 |     BITMAP CONVERSION FROM ROWIDS|                      |       |       |            |
|*  7 |      INDEX RANGE SCAN            | IND_TEST_OBJECT_NAME |       |       |     1   (0)|
----------------------------------------------------------------------------------------------

不带查询条件的情况,index combine将不被使用

SQL 10G>select/*+ index_combine(test ind_test_owner ind_test_object_name)*/ owner,object_name from test
  2  ;

Execution Plan
----------------------------------------------------------

-----------------------------------------------------------------------------------
| Id  | Operation                   | Name           | Rows  | Bytes | Cost (%CPU)|
-----------------------------------------------------------------------------------
|   0 | SELECT STATEMENT            |                | 51984 |  1472K|  1480   (1)|
|   1 |  TABLE ACCESS BY INDEX ROWID| TEST           | 51984 |  1472K|  1480   (1)|
|   2 |   INDEX FULL SCAN           | IND_TEST_OWNER | 51984 |       |   123   (2)|
-----------------------------------------------------------------------------------

index_combine会是and_equal的很好的替代者,随着and_equal的退出,index_combine将更多得被我们看到.