用示例说明B-Tree索引性能优于BitMap索引
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：
1 SQL> create table t_btree as select * from dba_objects; 2 3 Table created. 4 5 SQL> create index ind_tree on t_btree(object_id); 6 7 Index created.
执行两次下面的查询语句，并显示执行计划：
1 SQL> set autotrace traceonly; 2 SQL> select * from t_btree where object_id=9899; 3 4 5 Execution Plan 6 ---------------------------------------------------------- 7 Plan hash value: 447474086 8 9 ---------------------------------------------------------------------------------------- 10 | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | 11 ---------------------------------------------------------------------------------------- 12 | 0 | SELECT STATEMENT | | 1 | 207 | 2 (0)| 00:00:01 | 13 | 1 | TABLE ACCESS BY INDEX ROWID| T_BTREE | 1 | 207 | 2 (0)| 00:00:01 | 14 |* 2 | INDEX RANGE SCAN | IND_TREE | 1 | | 1 (0)| 00:00:01 | 15 ---------------------------------------------------------------------------------------- 16 17 Predicate Information (identified by operation id): 18 --------------------------------------------------- 19 20 2 - access("OBJECT_ID"=9899) 21 22 Note 23 ----- 24 - dynamic sampling used for this statement (level=2) 25 26 27 Statistics 28 ---------------------------------------------------------- 29 312 recursive calls 30 0 db block gets 31 108 consistent gets 32 289 physical reads 33 0 redo size 34 1404 bytes sent via SQL*Net to client 35 419 bytes received via SQL*Net from client 36 2 SQL*Net roundtrips to/from client 37 0 sorts (memory) 38 0 sorts (disk) 39 1 rows processed 40 41 SQL> select * from t_btree where object_id=9899; 42 43 44 Execution Plan 45 ---------------------------------------------------------- 46 Plan hash value: 447474086 47 48 ---------------------------------------------------------------------------------------- 49 | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | 50 ---------------------------------------------------------------------------------------- 51 | 0 | SELECT STATEMENT | | 1 | 207 | 2 (0)| 00:00:01 | 52 | 1 | TABLE ACCESS BY INDEX ROWID| T_BTREE | 1 | 207 | 2 (0)| 00:00:01 | 53 |* 2 | INDEX RANGE SCAN | IND_TREE | 1 | | 1 (0)| 00:00:01 | 54 ---------------------------------------------------------------------------------------- 55 56 Predicate Information (identified by operation id): 57 --------------------------------------------------- 58 59 2 - access("OBJECT_ID"=9899) 60 61 Note 62 ----- 63 - dynamic sampling used for this statement (level=2) 64 65 66 Statistics 67 ---------------------------------------------------------- 68 0 recursive calls 69 0 db block gets 70 4 consistent gets 71 0 physical reads 72 0 redo size 73 1404 bytes sent via SQL*Net to client 74 419 bytes received via SQL*Net from client 75 2 SQL*Net roundtrips to/from client 76 0 sorts (memory) 77 0 sorts (disk) 78 1 rows processed
接着创建跟t_btree一样的表t_bmap，并创建BitMap索引。
1 SQL> create table t_bmap as select * from dba_objects; 2 3 Table created. 4 5 SQL> create bitmap index ind_map on t_bmap(object_id); 6 7 Index created.
同样执行之前的语句两次：
1 SQL> select * from t_bmap where object_id=9899; 2 3 4 Execution Plan 5 ---------------------------------------------------------- 6 Plan hash value: 3763176822 7 8 ---------------------------------------------------------------------------------------- 9 | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | 10 ---------------------------------------------------------------------------------------- 11 | 0 | SELECT STATEMENT | | 1 | 207 | 110 (0)| 00:00:02 | 12 | 1 | TABLE ACCESS BY INDEX ROWID | T_BMAP | 1 | 207 | 110 (0)| 00:00:02 | 13 | 2 | BITMAP CONVERSION TO ROWIDS| | | | | | 14 |* 3 | BITMAP INDEX SINGLE VALUE | IND_MAP | | | | | 15 ---------------------------------------------------------------------------------------- 16 17 Predicate Information (identified by operation id): 18 --------------------------------------------------- 19 20 3 - access("OBJECT_ID"=9899) 21 22 Note 23 ----- 24 - dynamic sampling used for this statement (level=2) 25 26 27 Statistics 28 ---------------------------------------------------------- 29 312 recursive calls 30 0 db block gets 31 98 consistent gets 32 266 physical reads 33 0 redo size 34 1404 bytes sent via SQL*Net to client 35 419 bytes received via SQL*Net from client 36 2 SQL*Net roundtrips to/from client 37 0 sorts (memory) 38 0 sorts (disk) 39 1 rows processed 40 41 SQL> select * from t_bmap where object_id=9899; 42 43 44 Execution Plan 45 ---------------------------------------------------------- 46 Plan hash value: 3763176822 47 48 ---------------------------------------------------------------------------------------- 49 | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | 50 ---------------------------------------------------------------------------------------- 51 | 0 | SELECT STATEMENT | | 1 | 207 | 110 (0)| 00:00:02 | 52 | 1 | TABLE ACCESS BY INDEX ROWID | T_BMAP | 1 | 207 | 110 (0)| 00:00:02 | 53 | 2 | BITMAP CONVERSION TO ROWIDS| | | | | | 54 |* 3 | BITMAP INDEX SINGLE VALUE | IND_MAP | | | | | 55 ---------------------------------------------------------------------------------------- 56 57 Predicate Information (identified by operation id): 58 --------------------------------------------------- 59 60 3 - access("OBJECT_ID"=9899) 61 62 Note 63 ----- 64 - dynamic sampling used for this statement (level=2) 65 66 67 Statistics 68 ---------------------------------------------------------- 69 7 recursive calls 70 0 db block gets 71 68 consistent gets 72 0 physical reads 73 0 redo size 74 1404 bytes sent via SQL*Net to client 75 419 bytes received via SQL*Net from client 76 2 SQL*Net roundtrips to/from client 77 0 sorts (memory) 78 0 sorts (disk) 79 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。
