索引回表的优化

建立测试用表

create table t as select * from dba_objects;
create index idx_t_objectid on t(object_id);

查看执行计划

select * from t where object_id<10;
ZBB@test>select * from t where object_id<10;

8 rows selected.


Execution Plan
----------------------------------------------------------
Plan hash value: 3530610702

----------------------------------------------------------------------------------------------
| Id  | Operation                   | Name           | Rows  | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT            |                |     8 |  1656 |     3   (0)| 00:00:01 |
|   1 |  TABLE ACCESS BY INDEX ROWID| T              |     8 |  1656 |     3   (0)| 00:00:01 |
|*  2 |   INDEX RANGE SCAN          | IDX_T_OBJECTID |     8 |       |     2   (0)| 00:00:01 |
----------------------------------------------------------------------------------------------

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

   2 - access("OBJECT_ID"<10)

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


Statistics
----------------------------------------------------------
          0  recursive calls
          0  db block gets
          5  consistent gets
          0  physical reads
          0  redo size
       1921  bytes sent via SQL*Net to client
        523  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          8  rows processed

ZBB@test>

修改语句，修改显示查询列为必要的列，

select object_id from t where object_id<10;
ZBB@test>select object_id from t where object_id<10;

8 rows selected.


Execution Plan
----------------------------------------------------------
Plan hash value: 1876860048

-----------------------------------------------------------------------------------
| Id  | Operation        | Name           | Rows  | Bytes | Cost (%CPU)| Time     |
-----------------------------------------------------------------------------------
|   0 | SELECT STATEMENT |                |     8 |   104 |     2   (0)| 00:00:01 |
|*  1 |  INDEX RANGE SCAN| IDX_T_OBJECTID |     8 |   104 |     2   (0)| 00:00:01 |
-----------------------------------------------------------------------------------

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

   1 - access("OBJECT_ID"<10)

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


Statistics
----------------------------------------------------------
          0  recursive calls
          0  db block gets
          3  consistent gets
          0  physical reads
          0  redo size
        629  bytes sent via SQL*Net to client
        523  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          8  rows processed

ZBB@test>

通过以上的对比， 使用count(*)的时候，cost为3，逻辑读为5。仅仅查询object_id的时候，cost为2，逻辑读为3.
所以在查询的时候，避免不必要的列，还是很重要的。


-- 如果需要查询object_id,object_name列，但是只有object_id有索引，怎样避免回表？可以考虑使用object_id,object_name的组合列索引。

-- 查看不使用组合列索引的执行计划

select object_id,object_name from t where object_id<10;

ZBB@test>select object_id,object_name from t where object_id<10;

8 rows selected.


Execution Plan
----------------------------------------------------------
Plan hash value: 3530610702

----------------------------------------------------------------------------------------------
| Id  | Operation                   | Name           | Rows  | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT            |                |     8 |   632 |     3   (0)| 00:00:01 |
|   1 |  TABLE ACCESS BY INDEX ROWID| T              |     8 |   632 |     3   (0)| 00:00:01 |
|*  2 |   INDEX RANGE SCAN          | IDX_T_OBJECTID |     8 |       |     2   (0)| 00:00:01 |
----------------------------------------------------------------------------------------------

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

   2 - access("OBJECT_ID"<10)

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


Statistics
----------------------------------------------------------
          0  recursive calls
          0  db block gets
          5  consistent gets
          0  physical reads
          0  redo size
        768  bytes sent via SQL*Net to client
        523  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          8  rows processed

ZBB@test>

创建一个组合索引，查看执行计划，发现没有回表了 . cost从3变成了2，逻辑读从5变成了3 .

create index idx_object_id_name on t(object_id,object_name);
select object_id,object_name from t where object_id<10;

ZBB@test>select object_id,object_name from t where object_id<10;

8 rows selected.


Execution Plan
----------------------------------------------------------
Plan hash value: 1967165601

---------------------------------------------------------------------------------------
| Id  | Operation        | Name               | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT |                    |    14 |  1106 |     2   (0)| 00:00:01 |
|*  1 |  INDEX RANGE SCAN| IDX_OBJECT_ID_NAME |    14 |  1106 |     2   (0)| 00:00:01 |
---------------------------------------------------------------------------------------

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

   1 - access("OBJECT_ID"<10)

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


Statistics
----------------------------------------------------------
          0  recursive calls
          0  db block gets
          3  consistent gets
          0  physical reads
          0  redo size
        768  bytes sent via SQL*Net to client
        523  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          8  rows processed

ZBB@test>

END