Question

I’m using LINQ to Entities to display paged results. But I’m having issues with the combination of Skip(), Take() and OrderBy() calls.

Everything works fine, except that OrderBy() is assigned too late. It’s executed after result set has been cut down by Skip() and Take().

So each page of results has items in order. But ordering is done on a page handful of data instead of ordering of the whole set and then limiting those records with Skip() and Take().

How do I set precedence with these statements?

My example (simplified)

var query = ctx.EntitySet.Where(/* filter */).OrderByDescending(e => e.ChangedDate);
int total = query.Count();
var result = query.Skip(n).Take(x).ToList();

One possible (but a bad) solution

One possible solution would be to apply clustered index to order by column, but this column changes frequently, which would slow database performance on inserts and updates. And I really don’t want to do that.

EDIT

I ran ToTraceString() on my query where we can actually see when order by is applied to the result set. Unfortunately at the end. 🙁

SELECT 
-- columns
FROM  (SELECT 
    -- columns
    FROM   (SELECT -- columns
        FROM ( SELECT 
            -- columns
            FROM table1 AS Extent1
            WHERE  EXISTS (SELECT 
                -- single constant column
                FROM table2 AS Extent2
                WHERE (Extent1.ID = Extent2.ID) AND (Extent2.userId = :p__linq__4)
            )
        )  AS Project2
        limit 0,10  ) AS Limit1
    LEFT OUTER JOIN  (SELECT 
        -- columns
        FROM table2 AS Extent3 ) AS Project3 ON Limit1.ID = Project3.ID
UNION ALL
    SELECT 
    -- columns
    FROM   (SELECT -- columns
        FROM ( SELECT 
            -- columns
            FROM table1 AS Extent4
            WHERE  EXISTS (SELECT 
                -- single constant column
                FROM table2 AS Extent5
                WHERE (Extent4.ID = Extent5.ID) AND (Extent5.userId = :p__linq__4)
            )
        )  AS Project6
        limit 0,10  ) AS Limit2
    INNER JOIN table3 AS Extent6 ON Limit2.ID = Extent6.ID) AS UnionAll1
ORDER BY UnionAll1.ChangedDate DESC, UnionAll1.ID ASC, UnionAll1.C1 ASC

Answer 1

My workaround solution

I’ve managed to workaround this problem. Don’t get me wrong here. I haven’t solved precedence issue as of yet, but I’ve mitigated it.

What I did?

This is the code I’ve used until I get an answer from Devart. If they won’t be able to overcome this issue I’ll have to use this code in the end.

// get ordered list of IDs
List<int> ids = ctx.MyEntitySet
    .Include(/* Related entity set that is needed in where clause */)
    .Where(/* filter */)
    .OrderByDescending(e => e.ChangedDate)
    .Select(e => e.Id)
    .ToList();

// get total count
int total = ids.Count;

if (total > 0)
{
    // get a single page of results
    List<MyEntity> result = ctx.MyEntitySet
        .Include(/* related entity set (as described above) */)
        .Include(/* additional entity set that's neede in end results */)
        .Where(string.Format("it.Id in {{{0}}}", string.Join(",", ids.ConvertAll(id => id.ToString()).Skip(pageSize * currentPageIndex).Take(pageSize).ToArray())))
        .OrderByDescending(e => e.ChangedOn)
        .ToList();
}

First of all I’m getting ordered IDs of my entities. Getting only IDs is well performant even with larger set of data. MySql query is quite simple and performs really well. In the second part I partition these IDs and use them to get actual entity instances.

Thinking of it, this should perform even better than the way I was doing it at the beginning (as described in my question), because getting total count is much much quicker due to simplified query. The second part is practically very very similar, except that my entities are returned rather by their IDs instead of partitioned using Skip and Take…

Hopefully someone may find this solution helpful.