BitMap索引的效率要优于B-Tree索引实例

文章由LinuxBoy分享于2019-03-31 01:03:07热评（467）

BitMap索引的效率要优于B-Tree索引实例

一、实验说明：

操作系统：rhel 5.4 x86

数据库：Oracle 11g R2

二、操作步骤：

首先创建一张t_btree表，并建立B-Tree索引，索引键是status：

SQL> create table t_btree as select * from dba_objects;

Table created.

SQL> create index status_btree on t_btree(status);

Index created.

执行两次下面的查询语句，并显示执行计划：

SQL> set autotrace traceonly;
SQL> select count(*) from t_btree where status='VALID';

Execution Plan
----------------------------------------------------------
Plan hash value: 2400455617

--------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 5 | 49 (0)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | 5 | | |
|* 2 | INDEX FAST FULL SCAN| STATUS_BTREE | 74307 | 362K| 49 (0)| 00:00:01 |
--------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

2 - filter("STATUS"='VALID')

Note
-----
- dynamic sampling used for this statement (level=2)

Statistics
----------------------------------------------------------
32 recursive calls
0 db block gets
261 consistent gets
458 physical reads
0 redo size
424 bytes sent via SQL*Net to client
419 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed

SQL> select count(*) from t_btree where status='VALID';

Execution Plan
----------------------------------------------------------
Plan hash value: 2400455617

Predicate Information (identified by operation id):
---------------------------------------------------

2 - filter("STATUS"='VALID')

Note
-----
- dynamic sampling used for this statement (level=2)

Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
180 consistent gets
0 physical reads
0 redo size
424 bytes sent via SQL*Net to client
419 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed

接着创建跟t_btree一样的表t_bmap，并创建BitMap索引。

SQL> create table t_bmap as select * from dba_objects;

Table created.

SQL> create bitmap index status_bmap on t_bmap(status);

Index created.

同样执行之前的语句两次：

SQL> select count(*) from t_bmap where status='VALID';

Execution Plan
----------------------------------------------------------
Plan hash value: 516980546

---------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 5 | 3 (0)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | 5 | | |
| 2 | BITMAP CONVERSION COUNT | | 62928 | 307K| 3 (0)| 00:00:01 |
|* 3 | BITMAP INDEX FAST FULL SCAN| STATUS_BMAP | | | | |
---------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

3 - filter("STATUS"='VALID')

Note
-----
- dynamic sampling used for this statement (level=2)

Statistics
----------------------------------------------------------
32 recursive calls
0 db block gets
72 consistent gets
266 physical reads
0 redo size
424 bytes sent via SQL*Net to client
419 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed

SQL> select count(*) from t_bmap where status='VALID';

Execution Plan
----------------------------------------------------------
Plan hash value: 516980546

Predicate Information (identified by operation id):
---------------------------------------------------

3 - filter("STATUS"='VALID')

Note
-----
- dynamic sampling used for this statement (level=2)

Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
6 consistent gets
0 physical reads
0 redo size
424 bytes sent via SQL*Net to client
419 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed

从上面的查询中，我们可以得到，分别给两张内容一样的表做查询的时候，在执行第二次的时候是属于软解析：

从一致性读上比较，B-Tree索引的consistent gets是180，BitMap的是6；

从Cost的消耗上看，B-Tree索引的COST是49，而BitMap的是3。

在索引键是高重复率键值(status)的时候情况下BitMap索引的效率要优于B-Tree索引。

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