B-Tree索引性能优于BitMap索引实例

一、实验说明：

     操作系统：rhel 5.4 x86

    数据库：Oracle 11g R2

    实验说明：该实验是为了说明B-Tree索引性能优于BitMap索引的情况。 

二、实验操作：

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

SQL> create table t_btree as select * from dba_objects;

Table created.

SQL> create index ind_tree on t_btree(object_id);

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 * from t_btree where object_id=9899;

Execution Plan
----------------------------------------------------------
Plan hash value: 447474086

----------------------------------------------------------------------------------------
| Id  | Operation            | Name    | Rows  | Bytes | Cost (%CPU)| Time    |
----------------------------------------------------------------------------------------
|  0 | SELECT STATEMENT        |          |    1 |  207 |    2    (0)| 00:00:01 |
|  1 |  TABLE ACCESS BY INDEX ROWID| T_BTREE  |    1 |  207 |    2    (0)| 00:00:01 |
|*  2 |  INDEX RANGE SCAN        | IND_TREE |    1 |      |    1    (0)| 00:00:01 |
----------------------------------------------------------------------------------------

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

  2 - access("OBJECT_ID"=9899)

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

Statistics
----------------------------------------------------------
    312  recursive calls
      0  db block gets
    108  consistent gets
    289  physical reads
      0  redo size
  1404  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 * from t_btree where object_id=9899;

Execution Plan
----------------------------------------------------------
Plan hash value: 447474086

----------------------------------------------------------------------------------------
| Id  | Operation            | Name    | Rows  | Bytes | Cost (%CPU)| Time    |
----------------------------------------------------------------------------------------
|  0 | SELECT STATEMENT        |          |    1 |  207 |    2    (0)| 00:00:01 |
|  1 |  TABLE ACCESS BY INDEX ROWID| T_BTREE  |    1 |  207 |    2    (0)| 00:00:01 |
|*  2 |  INDEX RANGE SCAN        | IND_TREE |    1 |      |    1    (0)| 00:00:01 |
----------------------------------------------------------------------------------------

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

  2 - access("OBJECT_ID"=9899)

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

Statistics
----------------------------------------------------------
      0  recursive calls
      0  db block gets
      4  consistent gets
      0  physical reads
      0  redo size
  1404  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

同样执行之前的语句两次：

SQL>  select * from t_bmap where object_id=9899;

Execution Plan
----------------------------------------------------------
Plan hash value: 3763176822

----------------------------------------------------------------------------------------
| Id  | Operation            | Name    | Rows  | Bytes | Cost (%CPU)| Time    |
----------------------------------------------------------------------------------------
|  0 | SELECT STATEMENT        |          |    1 |  207 |  110    (0)| 00:00:02 |
|  1 |  TABLE ACCESS BY INDEX ROWID | T_BMAP  |    1 |  207 |  110    (0)| 00:00:02 |
|  2 |  BITMAP CONVERSION TO ROWIDS|          |      |      |        |          |
|*  3 |    BITMAP INDEX SINGLE VALUE | IND_MAP |      |      |        |          |
----------------------------------------------------------------------------------------

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

  3 - access("OBJECT_ID"=9899)

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

Statistics
----------------------------------------------------------
    312  recursive calls
      0  db block gets
    98  consistent gets
    266  physical reads
      0  redo size
  1404  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 * from t_bmap where object_id=9899;

Execution Plan
----------------------------------------------------------
Plan hash value: 3763176822

----------------------------------------------------------------------------------------
| Id  | Operation            | Name    | Rows  | Bytes | Cost (%CPU)| Time    |
----------------------------------------------------------------------------------------
|  0 | SELECT STATEMENT        |          |    1 |  207 |  110    (0)| 00:00:02 |
|  1 |  TABLE ACCESS BY INDEX ROWID | T_BMAP  |    1 |  207 |  110    (0)| 00:00:02 |
|  2 |  BITMAP CONVERSION TO ROWIDS|          |      |      |        |          |
|*  3 |    BITMAP INDEX SINGLE VALUE | IND_MAP |      |      |        |          |
----------------------------------------------------------------------------------------

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

  3 - access("OBJECT_ID"=9899)

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

Statistics
----------------------------------------------------------
      7  recursive calls
      0  db block gets
    68  consistent gets
      0  physical reads
      0  redo size
  1404  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4，BitMap的是68；

从Cost的消耗上看，B-Tree索引的COST是2，而BitMap的是110。

在索引键是主键或者唯一性约束的情况下B-Tree索引的效率要优于BitMap索引。