如何提高LINQ的性能？

Question

更新 感谢@usr我只需更改即可将此缩短至~3秒

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

至

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

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.有86个收集器记录,每个记录有50000+对应的日志记录.

我希望获得每个收集器的最新日志记录,这可以通过此SQL轻松完成

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

这需要大约2秒的时间来执行.

我能用LINQ得到的最接近的是以下内容

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,但需要大约35秒才能执行.

这成为以下SQL

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

如何才能使性能更接近单独使用SQL可实现的性能？

Answer 1

在原始 SQL 中，您可以从与 MAX(ID) 不同的行中选择集合 DateTimeUTC。这可能是一个错误。EF不存在这个问题。它在语义上并不相同，它是一个更难的查询。

如果您将 EF 查询重写为在结构上与 SQL 查询相同，您将获得相同的性能。我在这里看到 EF 不支持的内容。

也使用 EF计算max(id)并加入其中。