Why a simple linq command is not executed at the second case?

Question

I have the following line at my code.

var dates = query.Select(x => EntityFunctions.DiffDays(query.Min(y => y.Date), x.Date));

Howev

Answer 1

No it only does it once, by doing query.ToString() we can see the SQL Entity Framework is going to be generating. Your query generates the following SQL

SELECT 
    DATEDIFF (day, [GroupBy1].[A1], [Extent1].[Date]) AS [C1]
    FROM  [dbo].[Foos] AS [Extent1]
    CROSS JOIN  (SELECT 
        MIN([Extent2].[Date]) AS [A1]
        FROM [dbo].[Foos] AS [Extent2] ) AS [GroupBy1]

You can see it does a single query to get the minimum value then it joins the result of that query to your DATEDIFF where it will reuse that single result over and over.

---

The test program

class Context : DbContext
{
    public DbSet<Foo> Foo { get; set; }
}

public class Foo
{
    public int FooId { get; set; }
    public DateTime Date { get; set; }
}

class Program
{
    static void Main(string[] args)
    {
        using (var context = new Context())
        {
            var query = context.Foo;
            var dates = query.Select(x => EntityFunctions.DiffDays(query.Min(y => y.Date), x.Date));

            var result = dates.ToString();

            Debugger.Break();
        }
   } 
}