索引优化系列二 回表
http://blog.51cto.com/2853725/1421637
--索引回表读（TABLE ACCESS BY INDEX ROWID）的例子
drop table t purge;
create table t as select * from dba_objects;
create index idx1_object_id on t(object_id);
--试验1
set autotrace traceonly
set linesize 1000
set timing on
select * from t where object_id<=5;
执行计划
----------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
----------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 4 | 828 | 3 (0)| 00:00:01 |
| 1 | TABLE ACCESS BY INDEX ROWID| T | 4 | 828 | 3 (0)| 00:00:01 |
|* 2 | INDEX RANGE SCAN | IDX1_OBJECT_ID | 4 | | 2 (0)| 00:00:01 |
----------------------------------------------------------------------------------------------
统计信息
----------------------------------------------------------
0 recursive calls
0 db block gets
5 consistent gets
0 physical reads
0 redo size
1666 bytes sent via SQL*Net to client
415 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
4 rows processed
--比较消除TABLE ACCESS BY INDEX ROWID回表后的性能，将select * from改为select object_id from
set autotrace traceonly
set linesize 1000
set timing on
select object_id from t where object_id<=5;
执行计划
-----------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-----------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 4 | 52 | 2 (0)| 00:00:01 |
|* 1 | INDEX RANGE SCAN| IDX1_OBJECT_ID | 4 | 52 | 2 (0)| 00:00:01 |
-----------------------------------------------------------------------------------
统计信息
----------------------------------------------------------
0 recursive calls
0 db block gets
3 consistent gets
0 physical reads
0 redo size
478 bytes sent via SQL*Net to client
415 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
4 rows processed
--试验2：通过构造联合索引，再观察一个消除TABLE ACCESS BY INDEX ROWID的例子
set autotrace traceonly
set linesize 1000
select object_id,object_name from t where object_id<=5;
执行计划
----------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
----------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 4 | 316 | 3 (0)| 00:00:01 |
| 1 | TABLE ACCESS BY INDEX ROWID| T | 4 | 316 | 3 (0)| 00:00:01 |
|* 2 | INDEX RANGE SCAN | IDX1_OBJECT_ID | 4 | | 2 (0)| 00:00:01 |
----------------------------------------------------------------------------------------------
统计信息
----------------------------------------------------------
0 recursive calls
0 db block gets
5 consistent gets
0 physical reads
0 redo size
567 bytes sent via SQL*Net to client
415 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
4 rows processed
--准备工作，对t表建联合索引
create index idx_un_objid_objname on t(object_id,object_name);
--该联合索引建完后，产生功效了！消除了TABLE ACCESS BY INDEX ROWID
select object_id,object_name from t where object_id<=5;
执行计划
-----------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-----------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 12 | 948 | 2 (0)| 00:00:01 |
|* 1 | INDEX RANGE SCAN| IDX_UN_OBJID_OBJNAME | 12 | 948 | 2 (0)| 00:00:01 |
-----------------------------------------------------------------------------------------
统计信息
----------------------------------------------------------
0 recursive calls
0 db block gets
3 consistent gets
0 physical reads
0 redo size
567 bytes sent via SQL*Net to client
415 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
4 rows processed