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Everything You Ever Wanted to Know About SET NOCOUNT

SET NOCOUNT has to be one of the most used T-SQL statements. I can’t think of a single production script or procedure in which I didn’t use this SET statement. And I have to stretch to come up with a case when I would specifically not use it. The only time I don’t use NOCOUNT is when writing off-the-cuff test scripts. I decided to dig into it a bit and see what it really does behind the scenes.

SET NOCOUNT

From BOL:
“Stops the message that shows the count of the number of rows affected by a Transact-SQL statement or stored procedure from being returned as part of the result set.

SET NOCOUNT ON prevents the sending of DONE_IN_PROC messages to the client for each statement in a stored procedure. For stored procedures that contain several statements that do not return much actual data, or for procedures that contain Transact-SQL loops, setting SET NOCOUNT to ON can provide a significant performance boost, because network traffic is greatly reduced.”

I learned that the first statement isn’t as clear as it could be. I think it is technically true but easy to misunderstand. It appears to say that it stops the count from being returned in a result set. This is not true. The result still returns the count for statements that return data to the client. However, this count is marked as not to be used.

The second account is a bit misleading. It does stop the DONE_IN_PROC messages (tokens, actually, which I will explain later) from being sent. The reduction in network traffic is very case specific. Now it is always better to have less network traffic than more but I can think of only a few cases where a significant difference would be noticed.

Examining this requires a bit of understanding of Tabular Data Streams (TDS). I’m not interested in writing a series of columns about TDS at the moment so I’m going to keep this intro as short as possible. One of the great things about TDS is that we don’t have to know a lot about it ’cause it just works.
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