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执行计划出现BITMAP CONVERSION TO ROWIDS的解释!

原创 ERP 作者:rainbowbridg 时间:2007-10-17 15:28:29 0 删除 编辑

我在查看执行计划的时候发现BITMAP CONVERSION TO ROWIDS,好像sql反应比较慢,baidu了一下,有下面的解释!

我的解决办法是: 把索引删除,建立了联合索引,就没出现这个问题了,一致性获取也下降了很多,速度也快了不少!

不知道oracle用这个来究竟是有什么用!

http://www.itpub.net/showthread.php?s=&threadid=602866&perpage=10&pagenumber=1

都是cbo的context和算法决定的,可以试试false _b_tree_bitmap_plans,我说错了这个不应该是join是combine

给一段Jonathan Lewis的解释:
B-tree to Bitmap Conversions
One of the optimizer’s strategies is to range scan B-tree indexes to acquire lists of rowids, convert
the lists of rowids into the equivalent bitmaps, and perform bitwise operations to identify a
small set of rows. Effectively, the optimizer can take sets of rowids from index range scans and
convert them to bitmap indexes on the fly before doing an index_combine on the resulting
bitmap indexes.
In 8i, only tables with existing bitmap indexes could be subject to this treatment, unless
the parameter _b_tree_bitmap_plans had been set to relax the requirement for a preexisting
bitmap index.
In 9i, the default value for this parameter changed from false to true—so you may see
execution plans involving bitmap conversions after you’ve upgraded, even though you don’t
have a single bitmap index in your database. Unfortunately, because of the implicit packing
assumption that the optimizer uses for bitmap indexes, this will sometimes be a very bad idea.
As a related issue, this change can make it worth using the minimize_records_per_block
option on all your important tables.

其实,Lewis也给出了自己的疑问,认为将B树索引转换为除了在可选的方法里多了一种以外,不一定还有其他好处,甚至转换需要多消耗CPU资源。
呵呵,从我看来,这个功能好像不咋地。


有关bitwise,oracledba已经说得很详细了:
两个索引,分别转化为位图,满足各自条件的设置位为1,如果两个索引的位都为1,则rowid保留
上面这个过程就叫做bitwise operation.
BTW 查金山词霸,可以知道
bitwise logical operator
位逻辑运算


http://www.orawh.com/68.html

and_equal,index_join,index_combine
Published by wanghai六月 22, 2006 in Oracle.
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.

[@more@]

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