EF core 2 first query slow - linq

Question

I'm using EF core 2 as ORM in my project.
I faced this problem while executing this query:
var query = (from droitsGeo in _entities.DroitsGeos
join building in _entities.Batiments
on droitsGeo.IdPerimetre equals building.IdBatiment
where droitsGeo.IdUtilisateur == idUser &&
droitsGeo.IdClient == idClient &&
building.Valide == true &&
droitsGeo.IdNiveauPerimetre == geographicalLevel
orderby sort ascending
select new GeographicalModel
{
Id = building.IdBatiment,
IdParent = building.IdEtablissement,
Label = building.LibBatiment,
});
First execution tooks about 5 second and second less than one second as show below :
First execution of query :
Time elapsed EF: 00:00:04.8562419
After first execution of query :
Time elapsed EF: 00:00:00.5496862
Time elapsed EF: 00:00:00.6658079
Time elapsed EF: 00:00:00.6176030
I have same result using Stored procedure.
When i execute sql query generated by EF in SQL Server, the result is returned in less than a second.
what is wrong with EF Core 2 or did i miss something in configuration?

Answer 1

The EF by default tracks all the entities you run queries against.
When you run it for the first time the track change mechanism kicks in... that's why it takes a little bit longer.
You can avoid this, especially when retrieving collections by using .AsNoTracking() when composing the query.
Take a look:
var items = DbContext.MyDbSet
.Include(SecondObject)
.AsNoTracking()
.ToList();

Answer 2

EF core needs to compile LINQ quires using reflection therefor first queries are always slow. There is already a GitHub issue here

Answer 3

I have a simple idea to resolve this issue with the help of stored procedures and thereafter AutoMapper.
Create an stored procedures that return all the columns that you want, no matter if they are from different tables. Once the data is received from the stored procedure and you have received the object in one of your Model classes, you can then use AutoMapper to map only the relevant attributes to other classes. Please note that I am not giving you a tutorial of how to use stored procedure. I am giving you an example that might explain better:
A stored procedure is created which returns results from three tables named A, B and C.
A model class named SP_Result.cs is created corresponding to created stored procedure to map the received object of stored procedure (this is required when working with stored procedures in EF Core)
'ViewModels` are created having same attributes as returning from each table A, B and C.
Thereafter, mapping configurations will be created for SP_Result with ViewModel of Class A, Class B and Class C. e.g. CreateMap<SP_Result, ViewModel_A>(); CreateMap<SP_Result, ViewModel_B>();. I suppose, you would have a request and response objects which can be used instead of ViewModels. Name the properties accordingly in the stored procedure using AS keyword. e.g. Select std_Name AS 'Name'
This mapping will map the individual properties to each class. AutoMapper ignore the properties which do not exists in either of the classes mentioned in Mapping Configuration.
If you are selecting a list of objects where each object does have its own list of objects, this scenario will generally create N + 1 queries in EF. In fact, if you try to achieve this using stored procedures, you will have to create multiple queries or run the stored procedure multiple times (in a loop may be), or you will end up receiving Cartesian product.