I have a User table where there are a Username and Application columns. Username may repeat but combination of Username + Application is unique, but I don't have the unique constraint set on the table (for performance)
Question: will there be any difference (performance-wise) between :
SELECT * FROM User where UserName='myuser' AND Application='myapp'
AND -
SELECT TOP 1 * FROM User where UserName='myuser' AND Application='myapp'
As combination of Username + Application is unique, both queries will always return no more than one record, so TOP 1 doesn't affect the result. I always thought that adding TOP 1 will really speed things up as sql server would stop looking after it found one match, but I recently read in an article that using TOP will actually slow things down and it's recommended to avoid, though they haven't explained why.
Any comments?
Thank you! Andrey