Question

我的难题是尝试将以下T-SQL查询转换为近似等效（性能明智）的LINQ to SQL查询：

SELECT 
    j1.JOB,   
    max(CASE WHEN ISNULL(logs.statcategory, ' ') = 'PREP' THEN 'X' ELSE ' ' END) AS prep,  
    max(CASE WHEN ISNULL(logs.statcategory, ' ') = 'PRINT' THEN 'X' ELSE ' ' END) AS press,  
    max(CASE WHEN ISNULL(logs.statcategory, ' ') = 'BIND' THEN 'X' ELSE ' ' END) AS bind, 
    max(CASE WHEN ISNULL(logs.statcategory, ' ') = 'SHIP' THEN 'X' ELSE ' ' END) AS ship  
from 
    job j1  
left outer join 
( 
    select 
        j.job,
        l.statcategory,
        cnt=count(*)  
    from 
        job j  
    join 
        jobloc jl 
            join location l 
            on 
                l.code = jl.location and 
                l.site = jl.site 
        on j.job = jl.job  
    WHERE
        j.stat = 'O'  
    group by 
        j.job,l.statcategory  
) logs 
on 
    j1.job = logs.job  
WHERE
    j1.stat = 'O'  
group by
    j1.job

此查询目前在MS SQL Server上运行不到0.2秒。我提出的以下LINQ查询返回完全相同的记录，但运行速度慢了近30倍：

from a0 in Jobs
join a1 in 
(
    from a0 in Jobs
    join a1 in JobLocs on a0.Content equals a1.Job
    join a2 in Locations on new {Code = a1.Location, a1.Site} equals new {a2.Code, a2.Site}
    where a0.Stat == 'O'    
    select new {a0.Content, a2.StatCategory}
) on a0.Content equals a1.Content into a1
from a2 in a1.DefaultIfEmpty()
where a0.Stat == 'O'
group a2 by a0.Content into a0
orderby a0.Key
select new 
{
    Job = a0.Key,
    Prep = (bool?)a0.Max(a1 => a1.StatCategory == "PREP" ? true : false),
    Print = (bool?)a0.Max(a1 => a1.StatCategory == "PRINT" ? true : false),
    BIND = (bool?)a0.Max(a1 => a1.StatCategory == "BIND" ? true : false),
    SHIP = (bool?)a0.Max(a1 => a1.StatCategory == "SHIP" ? true : false),
}

以下是LINQ查询生成的SQL（使用LINQPad）：

-- Region Parameters
DECLARE @p0 Int = 79
DECLARE @p1 Int = 79
DECLARE @p2 VarChar(1000) = 'PREP'
DECLARE @p3 VarChar(1000) = 'PRINT'
DECLARE @p4 VarChar(1000) = 'BIND'
DECLARE @p5 VarChar(1000) = 'SHIP'
-- EndRegion
SELECT [t4].[Job], [t4].[value] AS [Prep], [t4].[value2] AS [Print], [t4].[value3] AS [BIND], [t4].[value4] AS [SHIP]
FROM (
    SELECT MAX(
        (CASE 
            WHEN [t3].[StatCategory] = @p2 THEN 1
            WHEN NOT ([t3].[StatCategory] = @p2) THEN 0
            ELSE NULL
         END)) AS [value], MAX(
        (CASE 
            WHEN [t3].[StatCategory] = @p3 THEN 1
            WHEN NOT ([t3].[StatCategory] = @p3) THEN 0
            ELSE NULL
         END)) AS [value2], MAX(
        (CASE 
            WHEN [t3].[StatCategory] = @p4 THEN 1
            WHEN NOT ([t3].[StatCategory] = @p4) THEN 0
            ELSE NULL
         END)) AS [value3], MAX(
        (CASE 
            WHEN [t3].[StatCategory] = @p5 THEN 1
            WHEN NOT ([t3].[StatCategory] = @p5) THEN 0
            ELSE NULL
         END)) AS [value4], [t0].[Job]
    FROM [Job] AS [t0]
    LEFT OUTER JOIN ([Job] AS [t1]
        INNER JOIN [JobLoc] AS [t2] ON [t1].[Job] = [t2].[Job]
        INNER JOIN [Location] AS [t3] ON ([t2].[Location] = [t3].[Code]) AND ([t2].[Site] = [t3].[Site])) ON ([t0].[Job] = [t1].[Job]) AND (UNICODE([t1].[Stat]) = @p0)
    WHERE UNICODE([t0].[Stat]) = @p1
    GROUP BY [t0].[Job]
    ) AS [t4]
ORDER BY [t4].[Job]

突出的一点是，LINQ查询生成的SQL运行子查询中返回的每个列的聚合，而在原始中它是外部SELECT的一部分。我可以想象部分性能下降是存在的。

我（暂时）愿意接受没有更好的方法来编写它，只需在LINQ API中使用DataContext.ExecuteQuery()方法（并直接运行和整形第一个SQL语句）。但是，我正在尝试不在我正在处理的项目中尽可能多地包含嵌入式SQL，因此如果它可以接近原始查询的性能，那将是理想的。我一直在讨论这段时间（部分是作为学术练习，也是实际使用这个或类似的查询），这是我提出的最好的（我没有写出原始查询顺便说一句 - 它是旧项目的一部分，正在迁移到更新的项目。）

感谢您的帮助。

Answer 1

根据我们在评论中的讨论，

问题在于linq-to-entities从某种未知原因添加的UNICODE转换。由于（不必要的）转换，DB无法使用索引。

您可以使用.Equals代替==，但不会使用UNICODE或将类型更改为数据库中的varchar(1)。

这个LINQ to SQL查询的更精炼版本

1 个答案: