Question

Based on this question:
What is difference between Where and Join in linq?

My question is following:

Is there a performance difference in the following two statements:

from order in myDB.OrdersSet
    from person in myDB.PersonSet
    from product in myDB.ProductSet
    where order.Persons_Id==person.Id && order.Products_Id==product.Id
    select new { order.Id, person.Name, person.SurName,  product.Model,UrunAdı=product.Name };

and

from order in myDB.OrdersSet
    join person in myDB.PersonSet on order.Persons_Id equals person.Id
    join product in myDB.ProductSet on order.Products_Id equals product.Id
    select new { order.Id, person.Name, person.SurName,  product.Model,UrunAdı=product.Name };

I would always use the second one just because it´s more clear.

My question is now, is the first one slower than the second one?
Does it build a cartesic product and filters it afterwards with the where clauses ?

Thank you.

Answer 1

It entirely depends on the provider you’re using.

With LINQ to Objects, it will absolutely build the Cartesian product and filter afterwards.

For out-of-process query providers such as LINQ to SQL, it depends on whether it’s smart enough to realise that it can translate it into a SQL join. Even if LINQ to SQL doesn’t, it’s likely that the query engine actually performing the query will do so – you’d have to check with the relevant query plan tool for your database to see what’s actually going to happen.

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Side-note: multiple “from” clauses don’t always result in a Cartesian product – the contents of one “from” can depend on the current element of earlier ones, e.g.

from file in files
from line in ReadLines(file)
...