我该如何提高这个LINQ的性能

本文关键字：LINQ 性能 何提高 | 更新日期: 2023-09-27 17:59:44

更新多亏了@usr，我只需更改就可以将时间缩短到大约3秒

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

至

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

我有一个数据库，有两个表——日志和收集器——我正在使用实体框架读取它们。共有86条收集器记录，每条记录都有50000多条相应的日志记录。

我想获得每个收集器的最新日志记录，这可以用这个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

这大约需要2秒钟才能执行。

我能用LINQ得到的最接近的是下面的

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，但执行大约需要35秒。

这将成为以下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

如何使性能更接近于单独使用SQL可以实现的性能？

我该如何提高这个LINQ的性能

在原始SQL中，您可以从与MAX（ID）不同的行中选择集合DateTimeUTC。这可能是个bug。外汇基金没有这个问题。它在语义上并不相同，这是一个更难查询的问题。

如果将EF查询重写为在结构上与SQL查询相同，那么您将获得相同的性能。我看不出英孚会不支持的。

计算具有EF的max(id)，并在此基础上加入。

我遇到了完全相同的问题，我通过添加索引解决了它。

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了。

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