sql - 使用联合优化 sql

Question

我尝试在临时表中插入数据，但无法让它工作。

我尝试从每次选择中选择 1000 个项目。

INSERT INTO #Data(user_id, created_at)
SELECT TOP(1000) u.id, c.created_at
FROM comment as c
inner join outfit as o on o.id = c.outfit_id and o.deleted = 0 and o.user_id = 645 AND c.deleted = 0
inner join user as u on u.id = c.user_id and u.is_active = 1

UNION

SELECT TOP(1000) u.id, l.created_at
FROM like as l
inner join outfit as o on o.id = l.outfit_id and o.deleted = 0 and o.user_id = 645
inner join user as u on u.id = l.user_id and u.is_active = 1

UNION 

SELECT TOP(1000) u.id, f.created_at
FROM friend_user as f
inner join user as u on u.id = f.user_id and u.is_active = 1
where f.friend_id = 645 AND f.approved = 1

UNION

SELECT TOP(1000) u.id, c.created_at
  FROM comment_tagged_user AS T
  INNER JOIN comment as c ON c.id = T.comment_id and T.user_id = 645 AND c.deleted = 0
  inner join outfit as o on o.id = c.outfit_id and o.deleted = 0
  inner join user as u on u.id = c.user_id and u.is_active = 1


ORDER BY o.created_at DESC

现在我尝试选择上面相同 sql 的总共 1000 行。 (我删除了每个 SELECT 上的 TOP 1000)

INSERT INTO #Data(user_id, created_at)
SELECT TOP 1000 * FROM ( 
   SELECT  u.id, c.created_at
   FROM comment as c
   inner join outfit as o on o.id = c.outfit_id and o.deleted = 0 and o.user_id = 645 AND c.deleted = 0
   inner join user as u on u.id = c.user_id and u.is_active = 1

   UNION

   SELECT u.id, l.created_at
   FROM like_like as l
   inner join outfit as o on o.id = l.outfit_id and o.deleted = 0 and o.user_id = 645
   inner join user as u on u.id = l.user_id and u.is_active = 1

   UNION 

   SELECT u.id, f.created_at
   FROM friend_user as f
   inner join user as u on u.id = f.user_id and u.is_active = 1
   where f.friend_id = 645 AND f.approved = 1

   UNION

   SELECT u.id, c.created_at
   FROM comment_tagged_user AS T
   INNER JOIN comment as c ON c.id = T.comment_id and T.user_id = 645 AND c.deleted = 0
   inner join outfit as o on o.id = c.outfit_id and o.deleted = 0
   inner join user as u on u.id = c.user_id and u.is_active = 1

) AS Data ORDER BY created_at DESC

但是结果并不一样。这就是我得到的:

左图来自第一个sql。第二张图显示了正确的结果。但是，第二条SQL需要7.8秒，第一条SQL只需要0.7秒。

那么，我在第一个sql中做错了什么？我不应该在列表的开头看到相同的结果吗？ 我使用 Azure Sql

Answer 1

对于 4000 行，您要对 4000 行进行排序，仅检索了 4 个表中每个表的前 1000 行，因此在第一个查询中，返回每个表的前 1000 行(不进行排序)，然后对这 4000 行结果执行排序。在第二个查询中，返回所有结果行，然后执行排序，仅返回前 1000 行。

这解释了输出的差异和性能的差异。