-- Key Seek index
CREATE  INDEX [IX dbo.Post thread_id, create_dt]
ON      dbo.Post (thread_id, create_dt);

DECLARE @PageNumber BIGINT,     -- Page number to fetch
        @PageSize   BIGINT;     -- Rows per page

SET     @PageSize = 50;
SET     @PageNumber = 10;

-- The thread_id to filter on
DECLARE @ThreadID   INTEGER;
SET     @ThreadID = 6;

-- Key-Seek algorithm
WITH    Keys
AS      (
        -- Step 1 : Number the rows from the non-clustered index
        -- Maximum number of rows = @PageNumber * @PageSize
        SELECT  TOP (@PageNumber * @PageSize)
                rn = ROW_NUMBER() OVER (ORDER BY P1.create_dt ASC),
                P1.post_id,
                P1.create_dt
        FROM    dbo.Post P1
        WHERE   P1.thread_id = @ThreadID
        ORDER   BY
                P1.create_dt ASC
        ),
        SelectedKeys
AS      (
        -- Step 2 : Get the primary keys for the rows on the page we want
        -- Maximum number of rows from this stage = @PageSize
        SELECT  TOP (@PageSize)
                SK.rn,
                SK.post_id,
                SK.create_dt
        FROM    Keys SK
        WHERE   SK.rn > ((@PageNumber - 1) * @PageSize)
        ORDER   BY
                SK.create_dt ASC
        )
SELECT  -- Step 3 : Retrieve the off-index data
        -- We will only have @PageSize rows by this stage
        SK.rn,
        P2.post_id,
        P2.thread_id,
        P2.member_id,
        P2.create_dt,
        P2.title,
        P2.body
FROM    SelectedKeys SK
JOIN    dbo.Post P2
        ON  P2.post_id = SK.post_id
ORDER   BY
        SK.create_dt ASC
OPTION  (RECOMPILE);