c# - 我怎样才能提高这个 LINQ 的性能？

Question

更新感谢@usr，我只需更改

就可以将其缩短到约 3 秒

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

到

    .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 Entity Framework 来读取它们。有86个收集器记录，每个收集器有50000+条对应的Log记录。

我想获取每个收集器的最新日志记录，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

这需要大约 2 秒的时间来执行。

我能得到的最接近 LINQ 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。

这就变成了下面的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

如何使性能更接近于仅使用 SQL 可实现的性能？

Answer 1

在您的原始 SQL 中，您可以从与 MAX(ID) 不同的行中选择一个集合 DateTimeUTC。这可能是一个错误。 EF没有这个问题。它在语义上并不相同，这是一个更难的查询。

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。我在这里看不到任何 EF 不支持的内容。

同时使用 EF 计算 max(id) 并加入。