Row_Number() CTE Performance when using ORDER BY CASE - sql-server

Question

I have a table I'd like to do paging and ordering on and was able to get a query similar to the following to do the work (the real query is much more involved with joins and such).
WITH NumberedPosts (PostID, RowNum) AS
(
SELECT PostID, ROW_NUMBER() OVER (ORDER BY
CASE WHEN #sortCol = 'User' THEN User END DESC,
CASE WHEN #sortCol = 'Date' THEN Date END DESC,
CASE WHEN #sortCol = 'Email' THEN Email END DESC) as RowNum
FROM Post
)
INSERT INTO #temp(PostID, User, Date, Email)
SELECT PostID, User, Date, Email
FROM Post
WHERE NumberedPosts.RowNum BETWEEN #start and (#start + #pageSize)
AND NumberedPosts.PostID = Post.PostID
The trouble is that performance is severely degraded when using the CASE statements (at least a 10x slowdown), when compared to a normal ORDER BY Date desc clause . Looking at the query plan it appears that all columns are still being sorted, even if they do not match the #sortCol qualifier.
Is there a way to get this to execute at near 'native' speed? Is dynamic SQL the best candidate for this problem? Thanks!

Answer 1

There shouldn't be any reason to query the post table twice. You can go the dynamic route and address those issues on performance or create 3 queries determined by the #sortCol parameter. Redundant code except for the row_num and order by parts, but sometimes you give up maintainability if speed is critical.
If #sortCol = 'User'
Begin
Select... Order by User
End
If #sortCol = 'Date'
Begin
Select .... Order by Date
end
If #sortCol = 'Email'
Begin
Select... Order by Email
End

Answer 2

Better to do this with either three hardcoded queries (in appropriate IF statements based on #sortCol) or dynamic SQL.
You might be able to do a trick with UNION ALL of three different queries (base on a base CTE which does all your JOINs), where only one returns rows for #sortCol, but I'd have to profile it before recommending it:
WITH BasePosts(PostID, User, Date, Email) AS (
SELECT PostID, User, Date, Email
FROM Posts -- This is your complicated query
)
,NumberedPosts (PostID, User, Date, Email, RowNum) AS
(
SELECT PostID, User, Date, Email, ROW_NUMBER() OVER (ORDER BY User DESC)
FROM BasePosts
WHERE #sortCol = 'User'
UNION ALL
SELECT PostID, User, Date, Email, ROW_NUMBER() OVER (ORDER BY Date DESC)
FROM BasePosts
WHERE #sortCol = 'Date'
UNION ALL
SELECT PostID, User, Date, Email, ROW_NUMBER() OVER (ORDER BY Email DESC)
FROM BasePosts
WHERE #sortCol = 'Email'
)
INSERT INTO #temp(PostID, User, Date, Email)
SELECT PostID, User, Date, Email
FROM NumberedPosts
WHERE NumberedPosts.RowNum BETWEEN #start and (#start + #pageSize)

Answer 3

I would definitely go down the dynamic SQL route (using sp_executesql with parameters to avoid any injection attacks). Using the CASE approach you're immediately stopping SQL Server from using any relevant indexes that would assist in the sorting process.

Answer 4

This should work, but not sure if it improves performance:
WITH NumberedPosts (PostID, RowNum) AS
(
SELECT PostID, ROW_NUMBER() OVER (ORDER BY
CASE WHEN #sortCol = 'User' THEN User
WHEN #sortCol = 'Date' THEN Date
WHEN #sortCol = 'Email' THEN Email
END DESC) as RowNum
FROM Post
)
INSERT INTO #temp(PostID, User, Date, Email)
SELECT PostID, User, Date, Email
FROM Post
WHERE NumberedPosts.RowNum BETWEEN #start and (#start + #pageSize)
AND NumberedPosts.PostID = Post.PostID