对 order by random() 的理解

开始

order by random() ，相当于不是根据某个字段来排序，而是乱序。这个方法可以帮助制造混乱分布的测试数据。

postgres=# \d employee

          Table "public.employee"

 Column |         Type          | Modifiers 

--------+-----------------------+-----------

 id     | integer               | 

 name   | character varying(20) | 

 deptno | integer               | 

 age    | integer               | 

Indexes:

    "idx_id_dept" btree (id, deptno)



postgres=# select name, age from employee order by name limit 5;

 name  | age 

-------+-----

 gao   |  30

 jian  |  35

 nam04 |  25

 nam05 |  40

 nam06 |  32

(5 rows)



postgres=# select name, age from employee order by age limit 5;

 name  | age 

-------+-----

 nam10 |  25

 nam16 |  25

 nam04 |  25

 nam28 |  25

 nam22 |  25

(5 rows)



postgres=# select name, age from employee order by random() limit 5;

  name  | age 

--------+-----

 nam560 |  40

 nam758 |  40

 nam635 |  40

 nam307 |  25

 nam708 |  30

(5 rows)



postgres=# select name, age from employee order by random() limit 5;

  name  | age 

--------+-----

 nam490 |  25

 nam444 |  32

 nam943 |  25

 nam143 |  35

 nam690 |  32

(5 rows)



postgres=# select name, age from employee order by 0.1 limit 5;

ERROR:  non-integer constant in ORDER BY

LINE 1: select name, age from employee order by 0.1 limit 5;

                                                ^

postgres=# select name, age from employee order by random() limit 5;

  name  | age 

--------+-----

 nam214 |  25

 nam474 |  32

 nam175 |  25

 nam22  |  25

 nam210 |  32

(5 rows)



postgres=# select name, age from employee order by random() limit 5;

  name  | age 

--------+-----

 nam390 |  30

 nam626 |  35

 nam342 |  30

 nam620 |  35

 nam674 |  35

(5 rows)



postgres=# select name, age from employee order by random() limit 5;

  name  | age 

--------+-----

 nam242 |  35

 nam847 |  30

 nam509 |  35

 nam945 |  32

 nam721 |  25

(5 rows)



postgres=# select name, age from employee order by random() limit 5;

  name  | age 

--------+-----

 nam165 |  32

 nam956 |  40

 nam446 |  35

 nam65  |  40

 nam291 |  30

(5 rows)



postgres=# 

[作者：技术者高健@博客园  mail: [email protected] ]

结束