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: luckyjackgao@gmail.com ]

结束