我怎样才能避免 xml 路径的多个使用内容？

问题描述

我在使用 sql 查询时遇到问题。我想要做的是获取商店列表，其中包含分配给用户的任务。每个角色在带有用户名的单独列中。我正在使用 STUFF 来获取连接行，但对于表中的 100K+ 行来说，这似乎是个坏主意。

这是简化的结构：

CREATE TABLE #temp_Users(
    [id] [int] IDENTITY(1,1) NOT NULL,[user_name] [nvarchar](250) NULL,CONSTRAINT [PK_Users] PRIMARY KEY CLUSTERED ([id] ASC)
 )

 insert into #temp_Users (user_name) values ('Joe'),('Jeff'),('Jimm')

任务表

create table #temp_Tasks (
    [id] [int] IDENTITY(1,[id_user] [int] NULL,[id_store] [int] NULL,[id_role] [int] NULL,CONSTRAINT [PK_Tasks] PRIMARY KEY CLUSTERED  ([id] ASC)
) 

insert into #temp_Tasks ([id_user],[id_store],[id_role])
       values (1,1,0),(1,2,(2,1),(3,1)

选择

SELECT distinct t.id_store,stuff( (SELECT ',' + u.[user_name] FROM #temp_Tasks t2 left outer join #temp_Users u on u.id = t2.id_user WHERE t2.id_store = t.id_store and t2.id_role = 0 FOR XML PATH('')),'' ) as 'role_0',' + u.[user_name] FROM #temp_Tasks t2 left outer join #temp_Users u on u.id = t2.id_user WHERE t2.id_store = t.id_store and t2.id_role = 1 FOR XML PATH('')),'' )  as 'role_1' 
FROM #temp_Tasks t

结果

问题是当表填充越来越多时，select 会变得越来越慢，因为每个 STUFF 都是另一个嵌套循环。如果我需要在这个选择中添加一些“角色”，我必须添加另一个像下面这样的东西：

SELECT distinct t.id_store,'' )  as 'role_1' 
  stuff( (SELECT ',' + u.[user_name] FROM #temp_Tasks t2 left outer join #temp_Users u on u.id = t2.id_user WHERE t2.id_store = t.id_store and t2.id_role = 2 FOR XML PATH('')),'' )  as 'role_2' 
  stuff( (SELECT ',' + u.[user_name] FROM #temp_Tasks t2 left outer join #temp_Users u on u.id = t2.id_user WHERE t2.id_store = t.id_store and t2.id_role = 3 FOR XML PATH('')),'' )  as 'role_3' 
  stuff( (SELECT ',' + u.[user_name] FROM #temp_Tasks t2 left outer join #temp_Users u on u.id = t2.id_user WHERE t2.id_store = t.id_store and t2.id_role = 4 FOR XML PATH('')),'' )  as 'role_4' 
FROM #temp_Tasks t

问题是我可以避免使用这多个 STUFF 函数吗？或者我可能需要在表任务上创建非聚集索引？也许每个角色都有过滤索引？提前致谢！

编辑：我使用的是 MSsql 2016。

正如 Gordon Linoff 建议的那样，我已将 Select 从 distinct 更改为 Group By，如下所示：

SELECT t.id_store,'' )  as 'role_1' 
    FROM #temp_Tasks t
group by t.id_store

并且执行时间从 20 秒减少到 2 秒。所以问题不在于 Stuff，而在于 distinct。

解决方法

从 SQL Server 2017 开始，您使用 string_agg()?

SELECT t.id_store,STRING_AGG(CASE WHEN t.id_role = 0 THEN u.user_name END,',') WITHIN GROUP (ORDER BY u.user_name) as role_0,STRING_AGG(CASE WHEN t.id_role = 1 THEN u.user_name END,') WITHIN GROUP (ORDER BY u.user_name) as role_1
FROM #temp_Tasks t JOIN
     #temp_Users u
     ON u.id = t2.id_users
GROUP BY t.id_store;

在旧版本中，您可能会受益于使用 GROUP BY 而不是 SELECT DISTINCT 作为 Aaron Bertrand explains。我也会用 LEFT JOIN:

替换子查询中的 INNER JOIN

SELECT t.id_store,stuff( (SELECT ',' + u.[user_name] FROM #temp_Tasks t2 join #temp_Users u on u.id = t2.id_user WHERE t2.id_store = t.id_store and t2.id_role = 0 FOR XML PATH('')),1,2,'' ) as 'role_0',' + u.[user_name] FROM #temp_Tasks t2 join #temp_Users u on u.id = t2.id_user WHERE t2.id_store = t.id_store and t2.id_role = 1 FOR XML PATH('')),'' )  as 'role_1' 
FROM #temp_Tasks t
GROUP BY t.id_store;

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