C# 如何改进此LINQ的性能？

更新 多亏了@usr，我只需改变一下，就可以把时间缩短到~3秒

.Select(
     log => log.OrderByDescending(
     d => d.DateTimeUTC
     ).FirstOrDefault()
)

到

我有一个包含两个表的数据库——日志和收集器——我正在使用实体框架来读取它们。共有86条收集器记录，每条记录有50000+条对应的日志记录

我想为每个收集器获取最新的日志记录，这很容易用这个SQL完成

SELECT CollectorLogModels_1.Status, CollectorLogModels_1.NumericValue,
    CollectorLogModels_1.StringValue, CollectorLogModels_1.DateTimeUTC,
    CollectorSettingsModels.Target, CollectorSettingsModels.TypeName 
FROM
    (SELECT CollectorId, MAX(Id) AS Id 
     FROM CollectorLogModels GROUP BY CollectorId) AS RecentLogs 
INNER JOIN CollectorLogModels AS CollectorLogModels_1 
   ON RecentLogs.Id = CollectorLogModels_1.Id 
      INNER JOIN CollectorSettingsModels 
          ON CollectorLogModels_1.CollectorId = CollectorSettingsModels.Id

执行此操作需要约2秒的时间

我能与LINQ取得的最接近的结果如下

var logs = context.Logs.Include(co => co.Collector)
                .GroupBy(
                    log => log.CollectorId, log => log
                )
                .Select(
                    log => log.OrderByDescending(
                        d => d.DateTimeUtc
                        ).FirstOrDefault()
                )
                .Join(
                    context.Collectors,
                    (l => l.CollectorId),
                    (c => c.Id),
                    (l, c) => new
                    {
                        c.Target,
                        DateTimeUTC = l.DateTimeUtc,
                        l.Status,
                        l.StringValue,
                        CollectorName = c.TypeName
                    }
                ).OrderBy(
                    o => o.Target
                ).ThenBy(
                    o => o.CollectorName
                )
                ;

这会产生我想要的结果，但执行需要约35秒

这将成为以下SQL语句

SELECT 
[Distinct1].[CollectorId] AS [CollectorId], 
[Extent3].[Target] AS [Target], 
[Limit1].[DateTimeUtc] AS [DateTimeUtc], 
[Limit1].[Status] AS [Status], 
[Limit1].[StringValue] AS [StringValue], 
[Extent3].[TypeName] AS [TypeName]
FROM    (SELECT DISTINCT 
    [Extent1].[CollectorId] AS [CollectorId]
    FROM [dbo].[CollectorLogModels] AS [Extent1] ) AS [Distinct1]
OUTER APPLY  (SELECT TOP (1) [Project2].[Status] AS [Status], [Project2].[StringValue] AS [StringValue], [Project2].[DateTimeUtc] AS [DateTimeUtc], [Project2].[CollectorId] AS [CollectorId]
    FROM ( SELECT 
        [Extent2].[Status] AS [Status], 
        [Extent2].[StringValue] AS [StringValue], 
        [Extent2].[DateTimeUtc] AS [DateTimeUtc], 
        [Extent2].[CollectorId] AS [CollectorId]
        FROM [dbo].[CollectorLogModels] AS [Extent2]
        WHERE [Distinct1].[CollectorId] = [Extent2].[CollectorId]
    )  AS [Project2]
    ORDER BY [Project2].[DateTimeUtc] DESC ) AS [Limit1]
INNER JOIN [dbo].[CollectorSettingsModels] AS [Extent3] ON [Limit1].[CollectorId] = [Extent3].[Id]
ORDER BY [Extent3].[Target] ASC, [Extent3].[TypeName] ASC

---

如何使性能更接近仅使用SQL即可实现的性能？

在原始SQL中，您可以从不同于MAX（ID）的行中选择集合DateTimeUTC。那可能是个错误。EF没有这个问题。它在语义上不完全相同，这是一个更难的查询

如果将EF查询重写为与SQL查询在结构上相同，则将获得相同的性能。我在这里没有看到EF不支持的内容

---

用EF计算

max（id）

，并加入其中。

我遇到了完全相同的问题，我通过添加索引解决了这个问题

---

我的一个查询需要45秒才能完成，我设法在不到一秒钟的时间内完成。

这只是一个想法，但也许你在ti有一个收获：如果你真的想获得最佳性能，你可以编写一个存储过程并通过EF处理它。描述工作原理：-1如果您接受了答案，则应创建一个新问题，而不是编辑您的问题。这使得usr的答案看起来是错误的。这是有道理的，因为DateTimeUTC不由Id isThanks索引，这是一个很大的帮助。我已经用结果更新了问题，现在缩短到~3秒！

SELECT 
[Distinct1].[CollectorId] AS [CollectorId], 
[Extent3].[Target] AS [Target], 
[Limit1].[DateTimeUtc] AS [DateTimeUtc], 
[Limit1].[Status] AS [Status], 
[Limit1].[StringValue] AS [StringValue], 
[Extent3].[TypeName] AS [TypeName]
FROM    (SELECT DISTINCT 
    [Extent1].[CollectorId] AS [CollectorId]
    FROM [dbo].[CollectorLogModels] AS [Extent1] ) AS [Distinct1]
OUTER APPLY  (SELECT TOP (1) [Project2].[Status] AS [Status], [Project2].[StringValue] AS [StringValue], [Project2].[DateTimeUtc] AS [DateTimeUtc], [Project2].[CollectorId] AS [CollectorId]
    FROM ( SELECT 
        [Extent2].[Status] AS [Status], 
        [Extent2].[StringValue] AS [StringValue], 
        [Extent2].[DateTimeUtc] AS [DateTimeUtc], 
        [Extent2].[CollectorId] AS [CollectorId]
        FROM [dbo].[CollectorLogModels] AS [Extent2]
        WHERE [Distinct1].[CollectorId] = [Extent2].[CollectorId]
    )  AS [Project2]
    ORDER BY [Project2].[DateTimeUtc] DESC ) AS [Limit1]
INNER JOIN [dbo].[CollectorSettingsModels] AS [Extent3] ON [Limit1].[CollectorId] = [Extent3].[Id]
ORDER BY [Extent3].[Target] ASC, [Extent3].[TypeName] ASC