How to speed up a slow update query in SQL Server 2012 - sql

Question

I have an update query that works fine, but it is way too slow and takes over 2 minutes to complete. Is there another way I can write this query to speed it up? Here is my code thanks:
UPDATE #tmpIMDS
SET
ModelFileName = b.ModelFileName,
SendEMail = b.SendEMail
FROM
(
SELECT DISTINCT
IMDSConversionReportData.ModelNumber,
ModelFileName,
'Send Email' AS SendEmail
FROM
IMDSConversionReportData,
(
SELECT DISTINCT
ModelNumber,
Max(DateAdded) AS DateAdded
FROM
IMDSConversionReportData
GROUP BY
ModelNumber) a
WHERE
IMDSConversionReportData.ModelNumber = a.ModelNumber
AND IMDSConversionReportData.DateAdded = a.DateAdded
) b
WHERE ModelID = b.ModelNumber

Answer 1

Instead of hitting IMDSConversionReportData table twice to get the maximum DateAdded per ModelNumber you can generate row_number to identify maximum DateAdded per ModelNumbercolumn.
Also remove distinct when you are selecting only one non aggregate column with group by which is meaningless
Try this
;WITH cte
AS (SELECT *,
'Send Email' AS SendEmail,
Row_number()OVER(partition BY ModelNumber ORDER BY DateAdded DESC) AS rn
FROM IMDSConversionReportData)
UPDATE t
SET ModelFileName = c.ModelFileName,
SendEMail = c.SendEMail
FROM #tmpIMDS t
INNER JOIN cte c
ON t.ModelID = c.ModelNumber
Where Rn = 1
Note : Always use proper INNER JOIN syntax to join two tables instead of Old style comma separated join. We always find INNER Join syntax is more readable. Keep the filters alone in Where clause