相关疑难解决方法(0)

正如标题所示,我想选择用a组成的每组行的第一行GROUP BY.

具体来说,如果我有一个purchases看起来像这样的表:

SELECT * FROM purchases;

我的输出:

id | customer | total
---+----------+------
 1 | Joe      | 5
 2 | Sally    | 3
 3 | Joe      | 2
 4 | Sally    | 1

我想查询每个产品id的最大购买量(total)customer.像这样的东西:

SELECT FIRST(id), customer, FIRST(total)
FROM  purchases
GROUP BY customer
ORDER BY total DESC;

预期产出:

FIRST(id) | customer | FIRST(total)
----------+----------+-------------
        1 | Joe      | 5
        2 | Sally    | 3

我在Postgres 9.2中有下表(简化形式)

CREATE TABLE log (
    log_date DATE,
    user_id  INTEGER,
    payload  INTEGER
);

它每个用户和每天最多包含一条记录.每天将有大约500,000条记录,为期300天.每个用户的running_total总是在增加.

我想在特定日期之前有效地检索每个用户的最新记录.我的查询是:

SELECT user_id, max(log_date), max(payload) 
FROM log 
WHERE log_date <= :mydate 
GROUP BY user_id

这非常慢.我也尝试过:

SELECT DISTINCT ON(user_id), log_date, payload
FROM log
WHERE log_date <= :mydate
ORDER BY user_id, log_date DESC;

具有相同的计划,同样缓慢.

到目前为止,我在user_msg_log(aggr_date)上有一个索引,但没有多大帮助.我应该用什么其他索引来加快速度,还是以任何其他方式实现我的目标？

有两个表conversations和messages，我想获取对话及其最新消息的内容。

conversations- id(主键)、名称、创建时间

messages- id、内容、created_at、conversation_id

目前我们正在运行此查询来获取所需的数据

SELECT
    conversations.id,
    m.content AS last_message_content,
    m.created_at AS last_message_at
FROM
    conversations
INNER JOIN messages m ON conversations.id = m.conversation_id
                     AND m.id = (
    SELECT
        id
    FROM
        messages _m
    WHERE
        m.conversation_id = _m.conversation_id
    ORDER BY
        created_at DESC
    LIMIT 1)
ORDER BY
    last_message_at DESC
LIMIT 15
OFFSET 0

上面的查询返回有效数据，但其性能随着行数的增加而降低。有没有其他方法可以提高性能来编写此查询？例如附加小提琴。

http://sqlfiddle.com/#!17/2decb/2

还尝试了已删除答案之一中的建议：

SELECT DISTINCT ON (c.id)
       c.id,
       m.content AS last_message_content,
       m.created_at AS last_message_at
  FROM conversations AS c
 INNER JOIN messages AS m …

我在表中有350万行acs_objects,我需要检索creation_date具有年份格式和不同的列.

我的第一次尝试:180~200 Sec (15 Rows Fetched)

SELECT DISTINCT to_char(creation_date,'YYYY') FROM acs_objects

我的第二次尝试:35~40 Sec (15 Rows Fetched)

SELECT DISTINCT to_char(creation_date,'YYYY')
FROM (SELECT DISTINCT creation_date FROM acs_objects) AS distinct_date

有没有办法让它更快？ - "我需要在ADP网站上使用它"

我有一个包含数亿行的表，我想从同一个表的 2 个索引列中获取唯一值的单个列表（没有唯一的行 ID）。

WITH cte as (
    SELECT fruit, veggie
    FROM recipes
)
SELECT fruit as healthy_food
         FROM cte
         UNION --  <--- 
         SELECT veggie as healthy_food
         FROM cte;

WITH cte as (...)
SELECT DISTINCT ON (healthy_food) healthy_food FROM  --  <--- 
(SELECT fruit as healthy_food
     FROM cte
     UNION ALL --  <--- 
     SELECT veggie as healthy_food
     FROM cte) tb;