【发布时间】：2011-11-29 02:31:45
【问题描述】：
我有一个非常繁重的 LINQ-to-SQL 查询，它对不同的表进行了许多连接以返回匿名类型。问题是，如果返回的行数相当大（> 200），那么查询会变得非常慢并最终超时。我知道我可以增加数据上下文超时设置，但这是最后的手段。
我只是想知道如果我将查询拆分后是否会更好，并将我的比较作为 LINQ-to-Objects 查询进行，这样我什至可以使用 PLINQ 来最大化处理能力。但我对我来说这是一个陌生的概念，我无法弄清楚我将如何拆分它。任何人都可以提供任何建议吗？我不是要求为我编写代码，只是一些关于如何改进它的一般指导会很棒。
请注意，我已确保数据库具有我要加入的所有正确密钥，并且我已确保这些密钥是最新的。
查询如下：
var cons = (from c in dc.Consignments
join p in dc.PODs on c.IntConNo equals p.Consignment into pg
join d in dc.Depots on c.DeliveryDepot equals d.Letter
join sl in dc.Accounts on c.Customer equals sl.LegacyID
join ss in dc.Accounts on sl.InvoiceAccount equals ss.LegacyID
join su in dc.Accounts on c.Subcontractor equals su.Name into sug
join sub in dc.Accountsubbies on ss.ID equals sub.AccountID into subg
where (sug.FirstOrDefault() == null
|| sug.FirstOrDefault().Customer == false)
select new
{
ID = c.ID,
IntConNo = c.IntConNo,
LegacyID = c.LegacyID,
PODs = pg.DefaultIfEmpty(),
TripNumber = c.TripNumber,
DropSequence = c.DropSequence,
TripDate = c.TripDate,
Depot = d.Name,
CustomerName = c.Customer,
CustomerReference = c.CustomerReference,
DeliveryName = c.DeliveryName,
DeliveryTown = c.DeliveryTown,
DeliveryPostcode = c.DeliveryPostcode,
VehicleText = c.VehicleReg + c.Subcontractor,
SubbieID = sug.DefaultIfEmpty().FirstOrDefault().ID.ToString(),
SubbieList = subg.DefaultIfEmpty(),
ScanType = ss.PODScanning == null ? 0 : ss.PODScanning
});
这是根据要求生成的 SQL：
{SELECT [t0].[ID], [t0].[IntConNo], [t0].[LegacyID], [t6].[test], [t6].[ID] AS [ID2], [t6].[Consignment], [t6].[Status], [t6].[NTConsignment], [t6].[CustomerRef], [t6].[Timestamp], [t6].[SignedBy], [t6].[Clause], [t6].[BarcodeNumber], [t6].[MainRef], [t6].[Notes], [t6].[ConsignmentRef], [t6].[PODedBy], (
SELECT COUNT(*)
FROM (
SELECT NULL AS [EMPTY]
) AS [t10]
LEFT OUTER JOIN (
SELECT NULL AS [EMPTY]
FROM [dbo].[PODs] AS [t11]
WHERE [t0].[IntConNo] = [t11].[Consignment]
) AS [t12] ON 1=1
) AS [value], [t0].[TripNumber], [t0].[DropSequence], [t0].[TripDate], [t1].[Name] AS [Depot], [t0].[Customer] AS [CustomerName], [t0].[CustomerReference], [t0].[DeliveryName], [t0].[DeliveryTown], [t0].[DeliveryPostcode], [t0].[VehicleReg] + [t0].[Subcontractor] AS [VehicleText], CONVERT(NVarChar,(
SELECT [t16].[ID]
FROM (
SELECT TOP (1) [t15].[ID]
FROM (
SELECT NULL AS [EMPTY]
) AS [t13]
LEFT OUTER JOIN (
SELECT [t14].[ID]
FROM [dbo].[Account] AS [t14]
WHERE [t0].[Subcontractor] = [t14].[Name]
) AS [t15] ON 1=1
ORDER BY [t15].[ID]
) AS [t16]
)) AS [SubbieID],
(CASE
WHEN [t3].[PODScanning] IS NULL THEN @p0
ELSE [t3].[PODScanning]
END) AS [ScanType], [t3].[ID] AS [ID3]
FROM [dbo].[Consignments] AS [t0]
INNER JOIN [dbo].[Depots] AS [t1] ON [t0].[DeliveryDepot] = [t1].[Letter]
INNER JOIN [dbo].[Account] AS [t2] ON [t0].[Customer] = [t2].[LegacyID]
INNER JOIN [dbo].[Account] AS [t3] ON [t2].[InvoiceAccount] = [t3].[LegacyID]
LEFT OUTER JOIN ((
SELECT NULL AS [EMPTY]
) AS [t4]
LEFT OUTER JOIN (
SELECT 1 AS [test], [t5].[ID], [t5].[Consignment], [t5].[Status], [t5].[NTConsignment], [t5].[CustomerRef], [t5].[Timestamp], [t5].[SignedBy], [t5].[Clause], [t5].[BarcodeNumber], [t5].[MainRef], [t5].[Notes], [t5].[ConsignmentRef], [t5].[PODedBy]
FROM [dbo].[PODs] AS [t5]
) AS [t6] ON 1=1 ) ON [t0].[IntConNo] = [t6].[Consignment]
WHERE ((NOT (EXISTS(
SELECT TOP (1) NULL AS [EMPTY]
FROM [dbo].[Account] AS [t7]
WHERE [t0].[Subcontractor] = [t7].[Name]
ORDER BY [t7].[ID]
))) OR (NOT (((
SELECT [t9].[Customer]
FROM (
SELECT TOP (1) [t8].[Customer]
FROM [dbo].[Account] AS [t8]
WHERE [t0].[Subcontractor] = [t8].[Name]
ORDER BY [t8].[ID]
) AS [t9]
)) = 1))) AND ([t2].[Customer] = 1) AND ([t3].[Customer] = 1)
ORDER BY [t0].[ID], [t1].[ID], [t2].[ID], [t3].[ID], [t6].[ID]
}
【问题讨论】：
-
后台执行的SQL是什么？查询计划是什么样的？
-
我会很好奇如果选择存储过程中的所有记录，使用 IMultilpleResults 调用它，然后在内存中进行连接不会更快。
-
您确定 Linq 是否在一个语句中完成所有操作（而不是每行选择的额外语句） - 不是 100% 确定在选择新场景中是否可能，所以像 @Heinzi 说的那样做：您需要查看使用分析器生成的 SQL 或记录您的数据上下文
-
我不确定 LINQ 是否适合您的场景。我会选择常规存储过程（我们打赌它要快多少？：p）。
-
虽然我喜欢 LINQ，但是当你有这样的复杂连接时，通常最好将其封装在单个存储过程中，然后通过 LINQ 获取存储过程的结果。另外，我可能错了，但真正的性能问题可能在您的 where 子句中。考虑将其更改为外连接（在 SQL 中）。
标签： c# linq linq-to-sql