组合索引的前缀与单列索引一致
drop table t purge;
create table t as select * from dba_objects;
create index idx_object_id on t(object_id,object_type);
set autotrace traceonly
set linesize 1000
--以下就能用到索引，因为object_id列是前缀
select * from t where object_id=19;
执行计划
---------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 207 | 3 (0)| 00:00:01 |
| 1 | TABLE ACCESS BY INDEX ROWID| T | 1 | 207 | 3 (0)| 00:00:01 |
|* 2 | INDEX RANGE SCAN | IDX_OBJECT_ID | 1 | | 2 (0)| 00:00:01 |
---------------------------------------------------------------------------------------------
统计信息
----------------------------------------------------------
0 recursive calls
0 db block gets
4 consistent gets
0 physical reads
0 redo size
1392 bytes sent via SQL*Net to client
416 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
组合索引的前缀与单列索引不一致
drop index idx_object_id;
create index idx_object_id on t(object_type, object_id);
--以下就不能用到索引，因为object_id列是后缀
select * from t where object_id=19;
执行计划
--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 12 | 2484 | 292 (1)| 00:00:04 |
|* 1 | TABLE ACCESS FULL| T | 12 | 2484 | 292 (1)| 00:00:04 |
--------------------------------------------------------------------------
统计信息
----------------------------------------------------------
0 recursive calls
0 db block gets
1049 consistent gets
0 physical reads
0 redo size
1389 bytes sent via SQL*Net to client
416 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
