table access by index rowid

table access by index rowid

回表：在数据中，当查询数据的时候，在索引中查找索引后，获得该行的rowid，根据rowid再查询表中数据，就是回表。

--创建一个表， 索引只建立在object_id上

SQL> create table ml_1 as
2  select * from dba_objects
3  ;

Table created

SQL> create index idx_ml_1 on ml_1 (object_id);

Index created

--当select语句只有object_id时， 就不存在回表，因为索引包含此列。

unicomidmp@UNICOM>select object_id from ml_1 where  object_id =46;

Execution Plan
----------------------------------------------------------
Plan hash value: 328998971

-----------------------------------------------------------------------------
| Id  | Operation	 | Name     | Rows  | Bytes | Cost (%CPU)| Time     |
-----------------------------------------------------------------------------
|   0 | SELECT STATEMENT |	    |	  1 |	 13 |	  1   (0)| 00:00:01 |
|*  1 |  INDEX RANGE SCAN| IDX_ML_1 |	  1 |	 13 |	  1   (0)| 00:00:01 |
-----------------------------------------------------------------------------

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

1 - access("OBJECT_ID"=46)

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

Statistics
----------------------------------------------------------
7  recursive calls
0  db block gets
69  consistent gets
0  physical reads
0  redo size
527  bytes sent via SQL*Net to client
524  bytes received via SQL*Net from client
2  SQL*Net roundtrips to/from client
0  sorts (memory)
0  sorts (disk)
1  rows processed

--当select不仅仅有object_id列，还有其他列， 这时就需要回表。

unicomidmp@UNICOM>select status,object_id from ml_1 where  object_id =46;

Execution Plan
----------------------------------------------------------
Plan hash value: 1099796311

----------------------------------------------------------------------------------------
| Id  | Operation		    | Name     | Rows  | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT	    |	       |     1 |    18 |     2	 (0)| 00:00:01 |
|   1 |  TABLE ACCESS BY INDEX ROWID| ML_1     |     1 |    18 |     2	 (0)| 00:00:01 |
|*  2 |   INDEX RANGE SCAN	    | IDX_ML_1 |     1 |       |     1	 (0)| 00:00:01 |
----------------------------------------------------------------------------------------

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

2 - access("OBJECT_ID"=46)

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

Statistics
----------------------------------------------------------
11  recursive calls
0  db block gets
72  consistent gets
0  physical reads
0  redo size
601  bytes sent via SQL*Net to client
524  bytes received via SQL*Net from client
2  SQL*Net roundtrips to/from client
0  sorts (memory)
0  sorts (disk)
1  rows processed

--所以并不是每次查询，都要走索引才是最优。

结论：

如果在obiect_id上建立索引，那么在执行该条SQL语句时，先进行索引扫描，在索引中找到 obiect_id=46 所在的位置（一般只需要扫描3个块数据即可）

，获得该行的ROWID，根据其ROWID再查询数据（回表），如果所查找的数据量较少，则回表次数就少。如上面的例子，

要查询的数据只有 obiect_id在索引中，status并不在索引中，那么就要回表一次查询status，如果status也在索引中，那么就不需要回表。

如果索引查询的数据量大， 那么回表的次数就多， 索引扫描的io块+回表的io > 全表扫描io ,此时就不能用索引。 即 no_index(x);

--end---

mengl 2016-5-22