TOP slows down query

Question

I have a pivot query on a table with millions of rows. Running the query normally, it runs in 2 seconds and returns 2983 rows. If I add TOP 1000 to the query it takes 10 secon

Answer 1

After doing some googling about suggesting an execution plan, I found the solution.

SELECT TOP 1000 * 
FROM (SELECT l.PatientID,
               l.LabID,
               l.Result
          FROM dbo.Labs l
          JOIN (SELECT MAX(LabDate) maxDate, 
                       PatientID, 
                       LabID 
                  FROM dbo.Labs 
              GROUP BY PatientID, LabID) s ON l.PatientID = s.PatientID
                                          AND l.LabID = s.LabID
                                          AND l.LabDate = s.maxDate) A
PIVOT(MIN(A.Result) FOR A.LabID IN ([1],[2],[3],[4],[5],[7],[8],[9],[10],[11],[12],[13],[14],[15],[16],[17])) p
OPTION (HASH JOIN)

OPTION (HASH JOIN) being the thing. The resulting execution plan for the version with TOP looks like the original non-top one, with a TOP tacked on at the end.

Since I was originally doing this in a view what I actually ended up doing was changing JOIN to INNER HASH JOIN