oracle 位图索引
创建数据：
CREATE TABLE TB_BITMAP
(
BITMAP_ID INTEGER NOT NULL,
SEX VARCHAR2(1) NOT NULL
);
BEGIN
FOR I IN 1..5000 LOOP
INSERT INTO TB_BITMAP (BITMAP_ID, SEX) VALUES (I, 'F');
END LOOP;
FOR I IN 5001..10000 LOOP
INSERT INTO TB_BITMAP (BITMAP_ID, SEX) VALUES (I, 'M');
END LOOP;
COMMIT;
END;
对于下面的查询：
select sex from tb_bitmap where sex='F';
如果创建B树索引：
SQL> create index idx_bitmap_sex on tb_bitmap(sex);
索引已创建。
SQL> set autotrace traceonly
SQL> select sex from tb_bitmap where sex='F';
已选择5000行。
执行计划
----------------------------------------------------------
Plan hash value: 920132054
-------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 5000 | 10000 | 6 (0)| 00:00:01 |
|* 1 | TABLE ACCESS FULL| TB_BITMAP | 5000 | 10000 | 6 (0)| 00:00:01 |
-------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("SEX"='F')
Note
-----
- dynamic sampling used for this statement
统计信息
----------------------------------------------------------
61 recursive calls
0 db block gets
401 consistent gets
5 physical reads
0 redo size
67719 bytes sent via SQL*Net to client
4048 bytes received via SQL*Net from client
335 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
5000 rows processed
SQL>
换成位图索引：
SQL> drop index idx_bitmap_sex;
索引已删除。
SQL> create bitmap index bmpidx_bitmap_sex on tb_bitmap(sex);
索引已创建。
SQL> select sex from tb_bitmap where sex='F';
已选择5000行。
执行计划
----------------------------------------------------------
Plan hash value: 3589321185
--------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 5000 | 10000 | 1 (0)| 00:00:01 |
| 1 | BITMAP CONVERSION TO ROWIDS | | 5000 | 10000 | 1 (0)| 00:00:01 |
|* 2 | BITMAP INDEX FAST FULL SCAN| BMPIDX_BITMAP_SEX | | | | |
--------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - filter("SEX"='F')
Note
-----
- dynamic sampling used for this statement
统计信息
----------------------------------------------------------
61 recursive calls
0 db block gets
43 consistent gets
0 physical reads
0 redo size
67719 bytes sent via SQL*Net to client
4048 bytes received via SQL*Net from client
335 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
5000 rows processed
SQL>
或者：
SQL> select * from tb_bitmap where sex='F';
已选择5000行。
执行计划
----------------------------------------------------------
Plan hash value: 346398197
--------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 5000 | 75000 | 6 (17)| 00:00:01 |
| 1 | TABLE ACCESS BY INDEX ROWID | TB_BITMAP | 5000 | 75000 | 6 (17)| 00:00:01 |
| 2 | BITMAP CONVERSION TO ROWIDS| | | | | |
|* 3 | BITMAP INDEX SINGLE VALUE | BMPIDX_BITMAP_SEX | | | | |
--------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
3 - access("SEX"='F')
Note
-----
- dynamic sampling used for this statement
统计信息
----------------------------------------------------------
7 recursive calls
0 db block gets
370 consistent gets
0 physical reads
0 redo size
98633 bytes sent via SQL*Net to client
4048 bytes received via SQL*Net from client
335 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
5000 rows processed
SQL>
此时如果还是B树索引：
SQL> drop index bmpidx_bitmap_sex;
索引已删除。
SQL> create index idx_bitmap_sex on tb_bitmap(sex);
索引已创建。
SQL> select * from tb_bitmap where sex='F';
已选择5000行。
执行计划
----------------------------------------------------------
Plan hash value: 920132054
-------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 5000 | 75000 | 6 (0)| 00:00:01 |
|* 1 | TABLE ACCESS FULL| TB_BITMAP | 5000 | 75000 | 6 (0)| 00:00:01 |
-------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("SEX"='F')
Note
-----
- dynamic sampling used for this statement
统计信息
----------------------------------------------------------
61 recursive calls
0 db block gets
401 consistent gets
5 physical reads
0 redo size
87629 bytes sent via SQL*Net to client
4048 bytes received via SQL*Net from client
335 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
5000 rows processed
SQL>
可见，对于distinct值比较少而且分布比较均匀的列，用位图索引比B树索引好。