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。 从一致性读

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。

Oracle B树索引简介（B-Tree Index）

浅析Oracle B-Tree index搜索原理

Oracle索引之B-Tree和Bitmap索引对比

从平衡树到oracle B-Tree索引的原理探索

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=99;

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"=99)

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

Statistics
----------------------------------------------------------
312 recursive calls
0 db block gets
108 consistent gets
2 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=99;

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"=99)

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=99;

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"=99)

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