Performance effect of using TOP 1 in a SELECT query

Question

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 con

Answer 1

you say you do not enforce the constraint, that translates there is no unique index on (UserName, Application) or (Application, UserName). Can the query use an access path that seeks either on UserName or Application? In other words, is any of these two columns indexed? If yes, then the plan will pick the most selective one which is indexed and do a range scan, possibly a nested loop with a bookmark lookup if the index is non-clustered, then a filter. Top 1 will stop the query after the first filter is matched, but whether this makes a difference depends on the cardinality of the data (how many records the range scan finds and how many satisfy the filter).

If there is no index then it will do a full clustered scan no matter what. Top 1 will stop the scan on first match, whether this is after processing 1 record or after processing 999 mil. records depdends on the actual user name and application...

The only thing that wil make a real difference is to allow the query to do a seek for both values, ie. have a covering index. The constraint would be enforced through exactly such a covering index. In other words: by turning off the constraint, presumably for write performance, be prepared to pay the price at reads. Is this read important? Did you do any measurement to confirm that the extra index write of the constraint would be critically dampening the performance?