PostgreSQL中的分组限制:显示每组的前N行？

Question

我需要为每个组取前N行,按自定义列排序.

鉴于下表:

db=# SELECT * FROM xxx;
 id | section_id | name
----+------------+------
  1 |          1 | A
  2 |          1 | B
  3 |          1 | C
  4 |          1 | D
  5 |          2 | E
  6 |          2 | F
  7 |          3 | G
  8 |          2 | H
(8 rows)

我需要每个section_id的前两行(按名称排序),即类似于的结果:

 id | section_id | name
----+------------+------
  1 |          1 | A
  2 |          1 | B
  5 |          2 | E
  6 |          2 | F
  7 |          3 | G
(5 rows)

我正在使用PostgreSQL 8.3.5.

Answer 1

新解决方案(PostgreSQL 8.4)

SELECT
  * 
FROM (
  SELECT
    ROW_NUMBER() OVER (PARTITION BY section_id ORDER BY name) AS r,
    t.*
  FROM
    xxx t) x
WHERE
  x.r <= 2;

Answer 2

从v9.3开始,你可以进行横向连接

select distinct t_outer.section_id, t_top.id, t_top.name from t t_outer
join lateral (
    select * from t t_inner
    where t_inner.section_id = t_outer.section_id
    order by t_inner.name
    limit 2
) t_top on true
order by t_outer.section_id;

它可能更快,但当然,您应该专门针对您的数据和用例测试性能.

Answer 3

横向联接是可行的方法，但您应该首先执行嵌套查询以提高大型表的性能。

SELECT t_limited.*
FROM (
        SELECT DISTINCT section_id
        FROM t
    ) t_groups
    JOIN LATERAL (
        SELECT *
        FROM t t_all
        WHERE t_all.section_id = t_groups.section_id
        ORDER BY t_all.name
        LIMIT 2
    ) t_limited ON true

如果没有嵌套选择不同，即使section_id经常重复，横向连接也会针对表中的每一行运行。使用嵌套选择不同，连接横向对于每个不同的section_id运行一次且仅一次。

Answer 4

这是另一个解决方案(PostgreSQL <= 8.3).

SELECT
  *
FROM
  xxx a
WHERE (
  SELECT
    COUNT(*)
  FROM
    xxx
  WHERE
    section_id = a.section_id
  AND
    name <= a.name
) <= 2