Linq to SQL：查询在SQL Server Management Studio中运行良好，但在应用程序上超时

Question

I'm maintaining the code of a legacy application. It is using Entity Framework 3.5 and updating it to a newer version at the moment is not possible.

There is a query that lately has started to time out. This is the query in LINQ:

var compoQueryResult = (from composition in MyAppDataContext.Compositions
                            join compositionStatus in MyAppDataContext.CompositionStatus on composition.StatusID equals compositionStatus.CompositionStatusID
                            join compoUnit in MyAppDataContext.CompositionUnits on composition.CompositionID equals compoUnit.CompositionID
                            join unit in MyAppDataContext.Units on compoUnit.UnitID equals unit.UnitID
                            join carUnit in MyAppDataContext.Car_Units on unit.UnitID equals carUnit.UnitId
                            join car in MyAppDataContext.Cars on carUnit.CarId equals car.CarID
                            join location in MyAppDataContext.Locations on unit.ParkedLocationID equals location.LocationID
                            from road in MyAppDataContext.Roads.Where(r => r.RoadID == unit.RoadID).DefaultIfEmpty()
                            from ta in MyAppDataContext.Allocations.Where(ta => ta.CompositionId == composition.CompositionID).DefaultIfEmpty()
                            from arrivalLocation in MyAppDataContext.Locations.Where(al => al.LocationID == ta.EndLocationID).DefaultIfEmpty()
                            where (MyApp.IsAdministrator || MyApp.GetLineIds().Contains(car.LineId.Value))
                            && car.LineId.Value == LineId                           
                            && (statusId == null || statusId == compositionStatus.CompositionStatusID)
                            && (locationId == null || unit.ParkedLocationID == locationId)
                            && (!onlyATMS.HasValue
                                 || (onlyATMS == true && composition.CompositionName.Contains(Common.Constants.ATMSSymbol))
                                 || (onlyATMS == false && !composition.CompositionName.Contains(Common.Constants.ATMSSymbol)))
                            select new CompositionQueryResultItem
                            {
                                CompositionId = composition.CompositionID,
                                CompositionName = composition.CompositionName,
                                CompositionNumber = composition.CompositionNumber,
                                DateTimeModified = composition.DataModified,
                                Status = compositionStatus.CompositionStatusDesc,
                                Location = location.LocationName,
                                Road = road.Name,
                                Number = ta.Number,
                                DepartureTime = ta.DepartTime,
                                ArrivalLocation = arrivalLocation.LocationName,
                                ArrivalTime = ta.ArriveTime
                            }).Distinct().ToList();

After debugging the app I can see that the following query is being executed:

SELECT DISTINCT [t10].[CompositionID] AS [CompositionId], [t10].[CompositionName], [t10].[CompositionNumber], [t10].[value] AS [DateTimeModified], [t10].[CompositionStatusDesc] AS [Status], [t10].[LocationName] AS [Location], [t10].[value2] AS [Road], [t10].[value3] AS [Number], [t10].[value4] AS [DepartureTime], [t10].[value5] AS [ArrivalLocation], [t10].[value6] AS [ArrivalTime]
FROM (
    SELECT [t0].[CompositionID], [t0].[CompositionName], [t0].[CompositionNumber], [t0].[DataModified] AS [value], [t1].[CompositionStatusDesc], [t6].[LocationName], [t7].[Name] AS [value2], [t8].[Number] AS [value3], [t8].[DepartTime] AS [value4], [t9].[LocationName] AS [value5], [t8].[ArriveTime] AS [value6], [t5].[LineId]
    FROM [dbo].[Composition] AS [t0]
    INNER JOIN [dbo].[CompositionStatus] AS [t1] ON [t0].[StatusID] = [t1].[CompositionStatusID]
    INNER JOIN [dbo].[CompositionUnit] AS [t2] ON [t0].[CompositionID] = [t2].[CompositionID]
    INNER JOIN [dbo].[Unit] AS [t3] ON [t2].[UnitID] = [t3].[UnitID]
    INNER JOIN [dbo].[Car_Unit] AS [t4] ON [t3].[UnitID] = [t4].[UnitId]
    INNER JOIN [dbo].[Car] AS [t5] ON [t4].[CarId] = [t5].[CarID]
    INNER JOIN [dbo].[Location] AS [t6] ON [t3].[ParkedLocationID] = ([t6].[LocationID])
    LEFT OUTER JOIN [dbo].[Road] AS [t7] ON ([t7].[RoadID]) = [t3].[RoadID]
    LEFT OUTER JOIN [dbo].[Allocation] AS [t8] ON [t8].[CompositionId] = ([t0].[CompositionID])
    LEFT OUTER JOIN [dbo].[Location] AS [t9] ON [t9].[LocationID] = [t8].[EndLocationID]
    ) AS [t10]
WHERE ([t10].[LineId]) = @p0

If I execute that query on SQL Server Manager Studio I get no issues at all. It executes in less than one second. However, the application is timing out and I have no idea why.

I tried doing

MyAppDataContext.ObjectTrackingEnabled = false;

but I still get the same error.

I'm about to give up and create an stored procedure instead, but I can't grasp why the timeout is happening.

Answer 1

The query execution plan that is being used by the SQL Server can be different when the same query is run through the application. Check your execution plans. You can use hints like option(recompile) at the end of the query or you can optimize it for a particular Lineid if for different id's the rows returned are significantly different. Sometimes, using such hints make it slow in SSMS but not in application. You can find a solution after some hit and trial.