Postgresql查询每组的前N条记录

表结构如下，

                Table "ytt.t1"
  Column |         Type          | Modifiers
 --------+-----------------------+-----------
  i_name | character varying(10) | not null
  rank   | integer               | not null

我模拟了20条数据来做演示。

 t_girl=# select * from t1 order by i_name;
  i_name  | rank
 ---------+------
  Charlie |   12
  Charlie |   12
  Charlie |   13
  Charlie |   10
  Charlie |   11
  Lily       |    6
  Lily       |    7
  Lily        |    7
  Lily       |    6
  Lily       |    5
  Lily    |    7
  Lily    |    4
  Lucy    |    1
  Lucy    |    2
  Lucy    |    2
  Ytt     |   14
  Ytt     |   15
  Ytt     |   14
  Ytt     |   14
  Ytt     |   15
 (20 rows)

.一、按照人名分组，每组取3条记录：

select * 
from (select i_name, rank, row_number() over(partition by i_name) as row from t1) t
where row < =3