Hello! I have this piece of SQL code: UPDATE a SET Field1 = c.Field1 FROM a INNER JOIN b ON a.GUID1 = b.GUID1 INNER JOIN c ON b.GUID2 = c.GUID2 WHERE c.Type = 1 AND @date BETWEEN b.DateFrom AND b.DateTo This query takes hours to complete. Now while trying to find out what's causing the poor performance (it surely looks simple enough!) I've rewritten it to use temp tables: SELECT a.GUID1, a.Field1, c.Type, b.DateFrom, b.DateTo INTO #temptable FROM a INNER JOIN b ON a.GUID1 = b.GUID1 INNER JOIN c ON b.GUID2 = c.GUID2 WHERE c.Type = 1 AND @date BETWEEN b.DateFrom AND b.DateTo UPDATE a SET Field1 = subsel.Field1 FROM (SELECT * FROM #temptable) AS subsel WHERE subsel.GUID1 = a.GUID1 Now it completes in 10 seconds. My question is why? Am I wrong in saying that the two batches above produce same results? Is there something I've missed about the UPDATE FROM syntax? Why would the first query perform THAT poorly? Table sizes: a: 24k rows b: 268k rows c: 260k rows GUIDs are of type uniqueidentifier. Any answers appreciated!