Linq Query 计数缓慢。 SQL查询非常快。我究竟做错了什么？

Question

我有一个非常基本的父子关系。对于我的主页，我只想从 children 表中获取计数。

var assignmenTotal = new AssignmentUser
{
    IsSupervisor = supervisor,
    AssignmentTotals = (
        from a in db.Assignments
        where (StartDate.HasValue) 
            ? DbFunctions.TruncateTime(a.CreatedDate) == StartDate 
            : a.IsArchived == false
        orderby a.ID ascending
        join b in db.Adjustments on a.ID equals b.AssignmentID
        group b by new {a.ID,a.UserName,a.Status,a.CreatedDate,a.IsArchived} 
        into g
        select new AssignmentTotals
        {
            ID =  g.Key.ID,
            UserName = g.Key.UserName,
            Status = g.Key.Status,
            ImportedDate = DbFunctions.TruncateTime(g.Key.CreatedDate),
            StartingLocation = (db.Adjustments
                .Where(x => x.AssignmentID == g.Key.ID)
                .OrderBy(x => x.LocationID)
                .Select(x => x.LocationID)
                .FirstOrDefault()),
            EndingLocation = (db.Adjustments.
                Where(x => x.AssignmentID == g.Key.ID)
                .OrderByDescending(x => x.LocationID)
                .Select(x => x.LocationID)
                .FirstOrDefault()),
            TotalLocations = g.Count(x => x.LocationID != null),
            TotalLicensePlates = g.Count(x => x.ExpectedLicensePlateID != null),
            TotalAdjCompleted = g.Count(x => x.Status == "C"),
            IsSameUser = (currUser == g.Key.UserName ? true : false),
            IsArchived = g.Key.IsArchived
        })
        .OrderBy(x => x.ID)
        .ToList()
};

现在展平的行总数约为 1000，这大约需要 10 秒才能完成。 如果我写一个 SQL 查询

SELECT ID, UserName, Status, b.StartLocation, b.EndLocation, b.TotalLocations, 
       b.TotalLicensePlates, b.TotalLocations 
FROM Assignments a
INNER JOIN(
SELECT AssignmentID,
min(LocationID) as StartLocation, max(LocationID) as EndLocation,
COUNT(CASE WHEN LocationID is NOT NULL THEN 1 ELSE 0 end) AS TotalLocations,
SUM(CASE WHEN ExpectedLicensePlateID IS NOT NULL THEN 1 ELSE 0 END )TotalLicensePlates,
SUM(CASE WHEN Status = 'C' THEN 1 ELSE 0 END )TotalAdjCompleted
FROM dbo.Adjustments
group by AssignmentID
) b on (a.ID = b.AssignmentID)
WHERE convert(date,a.CreatedDate) ='04/23/2021'

这需要不到一秒钟的时间来完成。

我认为我的问题出在 linq COUNT 部分。我试过做一个子查询，但仍然很慢。我认为问题在于 linq 查询将所有数据带到客户端并在客户端完成所有工作，而不是让服务器完成所有工作？

有没有更好的方法来做到这一点？

编辑：我正在使用实体框架，当我检查 SQL 分析器时，SQL 发送非常长且复杂。

Answer 1

这里的问题是，您编写了非等效的 LiNQ 查询。在投影中没有最佳预测如何处理FirstOrDefault。它会创建额外的 OUTER APPLY 连接，但速度很慢。

重写您的查询以尽可能接近 SQL：

 var query =
    from a in db.Assignments
    where (StartDate.HasValue) 
        ? DbFunctions.TruncateTime(a.CreatedDate) == StartDate 
        : a.IsArchived == false
    orderby a.ID ascending
    join b in db.Adjustments on a.ID equals b.AssignmentID
    group b by new {a.ID,a.UserName,a.Status,a.CreatedDate,a.IsArchived} 
    into g
    select new AssignmentTotals
    {
        ID =  g.Key.ID,
        UserName = g.Key.UserName,
        Status = g.Key.Status,
        ImportedDate = DbFunctions.TruncateTime(g.Key.CreatedDate),
        StartingLocation = g.Min(x => x.LocationID)
        EndingLocation = g.Max(x => x.LocationID),
        TotalLocations = g.Count(x => x.LocationID != null),
        TotalLicensePlates = g.Count(x => x.ExpectedLicensePlateID != null),
        TotalAdjCompleted = g.Count(x => x.Status == "C"),
        IsSameUser = (currUser == g.Key.UserName ? true : false),
        IsArchived = g.Key.IsArchived
    };

var totals = query
        .OrderBy(x => x.ID)
        .ToList();

var assignmenTotal = new AssignmentUser
{
    IsSupervisor = supervisor,
    AssignmentTotals = totals
};

Answer 2

实体框架（就像任何其他编程对象关系映射器一样）的性能和效率越差，您的请求越复杂。

所以，选项：

1. 处理它。嗯，这不是最好的。显然它会很慢，但如果它有效，它就会有效。
2. 在 EF EF6 / core 中使用原始 SQL 查询。
3. 使用不同的 ORM，但您需要自己评估它们以找出优缺点。
4. 完全放弃 C# 中的 SQL，并使用存储过程、视图、函数和其他 SQL 对象将 SQL 查询保留在数据库中。 （感谢Antonín Lejsek）