Question

I have a little program that needs to do some calculation on a data range. The range maybe contain about half a millon of records. I just looked to my db and saw that a group by was executed.

I thought that the result was executed on the first line, and later I just worked with data in RAM. But now I think that the query builder combine the expression.

var Test = db.Test.Where(x => x > Date.Now.AddDays(-7));
var Test2 = (from p in Test 
             group p by p.CustomerId into g 
             select new { UniqueCount = g.Count() } ); 

In my real world app I got more subqueries that is based on the range selected by the first query. I think I just added a big overhead to let the DB make different selects.

Now I bascilly just call .ToList() after the first expression.

So my question is am I right about that the query builder combine different IQueryable when it builds the expression tree?

Answer 1

Yes, you are correct. LINQ expressions are lazily evaluated at the moment you evaluate them (via .ToList(), for example). At that point in time, Entity Framework will look at the total query and build an SQL statement to represent it.

In this particular case, it’s probably wiser to not evaluate the first query, because the SQL database is optimized for performing set-based operations like grouping and counting. Rather than forcing the database to send all the Test objects across the wire, deserializing the results into in-memory objects, and then performing the grouping and counting locally, you will likely see better performance by having the SQL database just return the resulting Counts.