Question

我有这个查询，这是一个依赖查询并花费了很多执行时间

SELECT
  u.id,
  u.user_name,
  ifnull((select longitude from map where user_id = u.id order by map_id desc limit 1 ),0) as Longitude,
  ifnull((select latitude from map where user_id = u.id order by map_id desc limit 1 ),0) as Longitude,
  (select  created  from map  where user_id = 1  order by created desc   limit 1) as LatestTime
FROM users as u
WHERE id IN(SELECT
          user1_id FROM relation
        WHERE users.id = 1)
ORDER BY id;

我在（依赖）

中尝试了此查询

SELECT
  u.id,
  u.user_name,
  m.map_id,
  m.longitude,
  m.latitude,
  m.Date as created
FROM users as u
  left join (select
           map_id,
           longitude,
           latitude,
           user_id,
           max(created) as `Date`
         from map
         group by user_id) as m
    on m.user_id = u.id
WHERE id IN(SELECT
          user1_id FROM relation
        WHERE users.id = 1)
ORDER BY id;

问题是第一个查询依赖并且工作正常，但需要花费很多执行时间。对于第二个查询，问题是它没有获取最新创建的时间。现在我想优化这个查询。主题是在子查询中我首先创建组，然后我试图获得每组的最后记录。这是表结构。

users : id , user_name
map   : map_id ,  user_id ,longitude , latitude, created 
relations : id , user1_id , user2_id , relation

Answer 1

在需要性能的情况下，SELECT子句中的子查询确实很痛苦，必须放弃：）

您可以重写此部分：

SELECT
    u.id,
    u.user_name,
    ifnull((select longitude from map where user_id = u.id order by map_id desc limit 1 ),0) as Longitude,
    ifnull((select latitude from map where user_id = u.id order by map_id desc limit 1 ),0) as Longitude,
    (select  created  from map  where user_id = 1  order by created desc   limit 1) as LatestTime
FROM users as u

在：

SELECT
    u.id,
    u.user_name,
    COALESCE(m1.longitude, 0) as longitude,
    COALESCE(m1.latitude, 0) as latitude
FROM users u
LEFT JOIN map m1 ON m1.user_id = u.id
LEFT JOIN map m2 ON m2.user_id = m1.user_id AND m2.map_id > m1.map_id
WHERE m2.map_id IS NULL

我写了一个查询结构in this answer的简短说明。这是一个非常好的技巧，因为它更具可读性，无子查询和更高性能。

我还没有看过IN部分，但如果上述情况没有帮助的话。

编辑1：您可以提取创建的日期并改为使用MAX（）。

SELECT
    u.id,
    u.user_name,
    COALESCE(m1.longitude, 0) as longitude,
    COALESCE(m1.latitude, 0) as latitude,
    created.LatestTime
FROM (SELECT MAX(created) FROM map WHERE user_id = 1) created
INNER JOIN users u ON TRUE
LEFT JOIN map m1 ON m1.user_id = u.id
LEFT JOIN map m2 ON m2.user_id = m1.user_id AND m2.map_id > m1.map_id
WHERE m2.map_id IS NULL

优化子查询选择每个组的最后一条记录

1 个答案: