oracle同时取最大值与最小值sql语句性能测试

同时取最大最小值
--环境准备
drop table t purge;
create table t as select * from dba_objects;
update t set object_id=rownum;
commit;
alter table t add constraint pk_object_id primary key (OBJECT_ID);
set autotrace on
set linesize 1000
--看看执行计划是什么：
set linesize 1000
set autotrace on
select max(object_id),min(object_id) from t;
/*
执行计划
----------------------------------------------------------
Plan hash value: 1265209789

--------------------------------------------------------------------------------------
| Id  | Operation             | Name         | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT      |              |     1 |    13 |    40   (0)| 00:00:01 |
|   1 |  SORT AGGREGATE       |              |     1 |    13 |            |          |
|   2 |   INDEX FAST FULL SCAN| PK_OBJECT_ID | 54956 |   697K|    40   (0)| 00:00:01 |
--------------------------------------------------------------------------------------

Note
-----
   - dynamic sampling used for this statement


统计信息
----------------------------------------------------------
        145  recursive calls
          0  db block gets
        224  consistent gets
        142  physical reads
          0  redo size
        494  bytes sent via SQL*Net to client
        416  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          4  sorts (memory)
          0  sorts (disk)
          1  rows processed
*/
/*
小结: 执行计划并没有走高效的index full scan (min/max)扫描方式，而是走index fast full scan模式。
这是因为oracle不能同时在索引 相同的两段寻找最大值 和最小值 。
以上语句修改成以下利用笛卡尔积的查询方式完成.
*/
--同时取最大最小值的语句的改造写法
select max, min
from (select max(object_id) max from t ) a,
(select min(object_id) min from t ) b;
/*
执行计划
----------------------------------------------------------
Plan hash value: 3319831621

---------------------------------------------------------------------------------------------
| Id  | Operation                    | Name         | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT             |              |     1 |    26 |     4   (0)| 00:00:01 |
|   1 |  NESTED LOOPS                |              |     1 |    26 |     4   (0)| 00:00:01 |
|   2 |   VIEW                       |              |     1 |    13 |     2   (0)| 00:00:01 |
|   3 |    SORT AGGREGATE            |              |     1 |    13 |            |          |
|   4 |     INDEX FULL SCAN (MIN/MAX)| PK_OBJECT_ID | 54956 |   697K|     2   (0)| 00:00:01 |
|   5 |   VIEW                       |              |     1 |    13 |     2   (0)| 00:00:01 |
|   6 |    SORT AGGREGATE            |              |     1 |    13 |            |          |
|   7 |     INDEX FULL SCAN (MIN/MAX)| PK_OBJECT_ID | 54956 |   697K|     2   (0)| 00:00:01 |
---------------------------------------------------------------------------------------------

Note
-----
   - dynamic sampling used for this statement


统计信息
----------------------------------------------------------
          7  recursive calls
          0  db block gets
        118  consistent gets
          0  physical reads
          0  redo size
        472  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
*/
--性能在 consistent gets上得到了提升.