What's the difference between IOrderedQueryable and IQueryable?

Question

I have this:

    var points = from p in ContextDB.Points
                 orderby p.PointInTime descending
                 where p.InstanceID == instanceId
                 && p.ParentPointID == null
                 && p.PointTypeID == currentPointTypeID
                 select p;

and this:

    var points = from p in ContextDB.Points
                 where p.InstanceID == instanceId
                 && p.ParentPointID == null
                 && p.PointTypeID == currentPointTypeID
                 orderby p.PointInTime descending
                 select p;

While I understand the use of both (and one generates an error later) I don't understand how they are different.

I did see questions like this elsewhere on STO, but I've not garnered what the answer to this question is, I'm afraid.

Answer 1

A type implementing IOrderedQueryable<T> contains extra state to hold information about sorting.

IQueryable<T> normally represents an operation that will be performed later (and possibly in a completely different language on a different computer, e.g. with LINQ to SQL). A separate interface is needed because the next operation might be another sort, which needs to be treated differently to the first (to sort by column A and then B requires two LINQ operators to define, but the system needs to ensure that each key contributes to the overall sort).

Answer 2

It's easy to see if you translate the query comprehension to its corresponding Linq extension methods. The return type of OrderBy() is IOrderedEnumerable<>. Where() returns IEnumerable<>. What your first expression does is first sort all of the Points, then select only the ones that match the where clause. The last operation applied is Where, thus the expression type is IEnumerable<>.

Which one is more efficient depends largely on the Linq provider you use. My money is on sorting last. Although I'd guess that the provider is smart enough to order the operations in the best order. You might want to check that.