oracle 10g之 max 、min的写法

一、有主键情况

1、单查最大的主键object_id（cost:2、consistent gets:100）

select max(object_id) from t;

2、单查最小的主键object_id(cost:2、consistent gets:69)

select min(object_id) from t;

3、一起查max(object_id),min(object_id) (cost:47、consistent gets 224)

select max(object_id),min(object_id) from t;

4、最优一起查询(cost:4、consistent gets:138)

select max,min from (select max(object_id) max from t)a,(select min(object_id) min from t)b;

结论：

在字段为主键有索引的情况下，单查单个max、min效率最快

在字段为主键有索引的情况下，优化方式查询最快

在字段为主键有索引的情况下，直接max、min的方式最慢

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二、在t表中object_id不是主键没有唯一索引 并且有null值的情况

SQL> select max(object_id) from t;

MAX(OBJECT_ID)

--------------

178100

执行计划

----------------------------------------------------------

Plan hash value: 2966233522

---------------------------------------------------------------------------

| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |

---------------------------------------------------------------------------

| 0 | SELECT STATEMENT | | 1 | 13 | 288 (1)| 00:00:04 |

| 1 | SORT AGGREGATE | | 1 | 13 | | |

| 2 | TABLE ACCESS FULL| T | 69433 | 881K| 288 (1)| 00:00:04 |

---------------------------------------------------------------------------

Note

-----

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

统计信息

----------------------------------------------------------

48 recursive calls

0 db block gets

1099 consistent gets

1024 physical reads

0 redo size

343 bytes sent via SQL*Net to client

350 bytes received via SQL*Net from client

2 SQL*Net roundtrips to/from client

0 sorts (memory)

0 sorts (disk)

1 rows processed

SQL> select min(object_id) from t;

MIN(OBJECT_ID)

--------------

2

执行计划

----------------------------------------------------------

Plan hash value: 2966233522

---------------------------------------------------------------------------

| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |

---------------------------------------------------------------------------

| 0 | SELECT STATEMENT | | 1 | 13 | 288 (1)| 00:00:04 |

| 1 | SORT AGGREGATE | | 1 | 13 | | |

| 2 | TABLE ACCESS FULL| T | 69433 | 881K| 288 (1)| 00:00:04 |

---------------------------------------------------------------------------

Note

-----

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

统计信息

----------------------------------------------------------

4 recursive calls

0 db block gets

1095 consistent gets

0 physical reads

0 redo size

342 bytes sent via SQL*Net to client

350 bytes received via SQL*Net from client

2 SQL*Net roundtrips to/from client

0 sorts (memory)

0 sorts (disk)

1 rows processed

SQL> select max(object_id),min(object_id) from t;

MAX(OBJECT_ID) MIN(OBJECT_ID)

-------------- --------------

178100 2

执行计划

----------------------------------------------------------

Plan hash value: 2966233522

---------------------------------------------------------------------------

| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |

---------------------------------------------------------------------------

| 0 | SELECT STATEMENT | | 1 | 13 | 288 (1)| 00:00:04 |

| 1 | SORT AGGREGATE | | 1 | 13 | | |

| 2 | TABLE ACCESS FULL| T | 69433 | 881K| 288 (1)| 00:00:04 |

---------------------------------------------------------------------------

Note

-----

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

统计信息

----------------------------------------------------------

4 recursive calls

0 db block gets

1095 consistent gets

0 physical reads

0 redo size

406 bytes sent via SQL*Net to client

350 bytes received via SQL*Net from client

2 SQL*Net roundtrips to/from client

0 sorts (memory)

0 sorts (disk)

1 rows processed

SQL> select max,min from (select max(object_id) max from t) a,(select min(object_id) min from t) b;

MAX MIN

---------- ----------

178100 2

执行计划

----------------------------------------------------------

Plan hash value: 1937292215

-----------------------------------------------------------------------------

| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |

-----------------------------------------------------------------------------

| 0 | SELECT STATEMENT | | 1 | 26 | 575 (1)| 00:00:07 |

| 1 | NESTED LOOPS | | 1 | 26 | 575 (1)| 00:00:07 |

| 2 | VIEW | | 1 | 13 | 288 (1)| 00:00:04 |

| 3 | SORT AGGREGATE | | 1 | 13 | | |

| 4 | TABLE ACCESS FULL| T | 69433 | 881K| 288 (1)| 00:00:04 |

| 5 | VIEW | | 1 | 13 | 288 (1)| 00:00:04 |

| 6 | SORT AGGREGATE | | 1 | 13 | | |

| 7 | TABLE ACCESS FULL| T | 69433 | 881K| 288 (1)| 00:00:04 |

-----------------------------------------------------------------------------

Note

-----

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

统计信息

----------------------------------------------------------

7 recursive calls

0 db block gets

2190 consistent gets

0 physical reads

0 redo size

384 bytes sent via SQL*Net to client

350 bytes received via SQL*Net from client

2 SQL*Net roundtrips to/from client

0 sorts (memory)

0 sorts (disk)

1 rows processed

结论：

优化的方式查询还没有一起查询快，一起查询及单个查询花费的cpu几乎一样多。

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最终结论:

本实验为看别人思路模拟的本地环境测试，表明需要根据实际场景进行sql优化，不一定在某个环境及情况下优化过的sql就永远是最好的。