[英]How can i improve the performance of this LINQ?
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?
在原始SQL中,您可以从与MAX(ID)不同的行中选择一个集合DateTimeUTC。 那可能是个错误。 EF没有那个问题。 它在语义上不相同,这是一个更难的查询。
如果您重写EF查询在结构上与SQL查询相同,您将获得相同的性能。 我在这里看不到EF不支持的任何内容。
用EF计算max(id)
并加入。
我有完全相同的问题,我通过添加索引解决了它。
我的查询需要45秒才能完成,我设法在不到一秒的时间内完成。
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