sql - 从 select 语句推断的 Postgres 隐式类型

Question

我问了一个similar questions yesterday关于 postgress，以及它是否可以从 select 语句的结果形状推断出类型。

今天我想从查询返回一个结果集，这是我发现有效的查询:

DROP TYPE IF EXISTS topic_result_entry CASCADE;
CREATE TYPE topic_result_entry AS
(
    id            INTEGER,  
    last_post_at  TIMESTAMP WITHOUT TIME ZONE,
    is_sticky     BOOLEAN,
    is_poll       BOOLEAN,
    has_prefix    BOOLEAN,
    prefix        CHARACTER VARYING,
    title         CHARACTER VARYING,
    post_count    INTEGER,
    started_by    INTEGER,
    started_at    TIMESTAMP WITHOUT TIME ZONE
);

CREATE OR REPLACE FUNCTION get_paginated_topics(
    forum_id_ INTEGER, category_id_ INTEGER, page_number_ INTEGER, topics_per_page_ INTEGER)
RETURNS SETOF topic_result_entry as $$
DECLARE
    zero_based_index INTEGER;
    lower_offset INTEGER;
    upper_offset INTEGER;

BEGIN                    
    zero_based_index := page_number_ -1;
    lower_offset := zero_based_index * topics_per_page_;
    upper_offset := ( (topics_per_page_ * page_number_) + 1 );               

    RETURN query
    select id,last_post_at, is_sticky, is_poll, 
           has_prefix, prefix, title,post_count, 
           started_by, started_at 
    from (
        select row_number() OVER(ORDER by last_post_at desc) as rn, * 
        from forum_topics where category_id = category_id_ and forum_id= forum_id_
    ) as foo
    where rn > lower_offset and rn < upper_offset;
END;
$$ LANGUAGE plpgsql;

结果集的形状可以从select的参数列表+源表的schema定义中推断出来。

Q1。 9.1 中是否有一些语法糖，如果没有，是否在路线图上？

Q2 是否有更简洁的方式来做到这一点？

离题

select id,last_post_at, is_sticky, is_poll, 
       has_prefix, prefix, title,post_count, 
       started_by, started_at 
from (
    select row_number() OVER(ORDER by last_post_at desc) as rn, * 
    from forum_topics where category_id = 72
) as foo
where rn>0 and rn<22

QUERY PLAN  
Subquery Scan on foo  (cost=0.00..492.20 rows=28 width=60)  
  Filter: ((foo.rn > 0) AND (foo.rn < 22))  
  ->  WindowAgg  (cost=0.00..409.42 rows=5519 width=156)    
        ->  Index Scan using forum_topics_last_post_at_idx1 on forum_topics  (cost=0.00..326.63 rows=5519 width=156)    
              Filter: (category_id = 72)

Answer 1

语法糖？我给你买了一些。

如果……

The shape of the resultset can be inferred from [...] the schema definition of the source table

...那么你可以大大简化。在 PostgreSQL 中，表定义会自动定义一个同名的类型。

CREATE OR REPLACE FUNCTION get_paginated_topics(
    _forum_id int, _category_id int, _page_number int, _topics_per_page int)
RETURNS <b>SETOF forum_topics</b> AS
$BODY$
DECLARE
   _lower_offset  int := (_page_number - 1) * _topics_per_page;
   _upper_offset  int := _topics_per_page * _page_number + 1;
BEGIN

   RETURN QUERY
   SELECT *
   FROM   forum_topics f
   WHERE  f.category_id = category_id_
   AND    f.forum_id = forum_id_
   ORDER  BY f.last_post_at DESC
   LIMIT  _lower_offset
   OFFSET _upper_offset;

END;
$BODY$ LANGUAGE plpgsql;

其他细节:

• 按照@user272735 指出的那样使用 LIMIT/OFFSET。

  • 然后你可以使用 SELECT *，因为你去掉了多余的列。

• 您可以在声明时分配变量。

• 短类型名称。

• 删除了多余的括号。

• 我在查询中添加了表别名和表限定，这在本例中不是必需的，但这是避免 plpgsql 函数中命名冲突的良好做法。