September 3, 2002 at 12:00 am
Comments posted to this topic are about the content posted at http://www.sqlservercentral.com/columnists/hroggero/usingbitmaskoperators.asp
Herve Roggero
hroggero@pynlogic.com
MCDBA, MCSE, MCSD
SQL Server Database Proxy/Firewall and Auditing
September 5, 2002 at 2:28 am
Thanks for raising another interesting topic. I had never considered the performance benefits of doing things this way, but have used the method for other reasons (sometimes you simply don't need a reference data table).
Anyway, using your example schema, I just thought I'd highlight the fact that the "traditional" result set of all errors for a/all users can also be generated using the bitwise method.
Cheers,
Daniel
***************************************
DECLARE @user-id int
SET @UserId = 3
-- return all errors for a user using the bitwise method in a join
SELECT U.*, E.*
FROM Users U
JOIN Errors E
ON E.ErrorFlag & U.UserErrorFlag = E.ErrorFlag
WHERE U.Userid = @UserId
-- return all errors for a user using a join to the many-to-many intermediate table
SELECT U.*, E.*
FROM Users U
JOIN UserErrors UE
ON UE.UserId = U.UserId
JOIN Errors E
ON E.ErrorId = UE.ErrorId
WHERE U.UserId = @UserId
September 6, 2002 at 7:07 pm
Daniel, thanks for your input and your feedback.
I agree with you. Actually, I have seen cases where so many locks exist on small and static tables that the application layer was really slowing down.
I guess the only danger of not using referenced tables would be data integrity. I usually enforce integrity with the reference table, but use bitwise operators to get to the data, when it makes sense.
Herve
Herve Roggero
hroggero@pynlogic.com
MCDBA, MCSE, MCSD
SQL Server Database Proxy/Firewall and Auditing
December 1, 2005 at 8:05 pm
Nice article... you really put some thought into the proofs and which graphics to use. Good idea, as well.
--Jeff Moden
Change is inevitable... Change for the better is not.
July 13, 2007 at 9:29 am
It's interesting you did not point out the possibility of a "double unknown" of using this method. That is, it's possible for the error field to be either 0 or NULL. Both values would have, essentially, the same meaning. It's possible to make use of this fact, say 0 is success and NULL is the program has reported back yet.
But, I would really love to know more, specifically, how large your dictionary can be? This is complicated by the fact that MS stores your larger values signed. For example, a TINYINT is unsigned and can store 7 concurrent values in addition to NULL and 0. But in order to expand beyond that using SMALLINT, you have to divide the value between positive and negative, -2^15 (-32,768) to 2^15-1 (32,767). This gives you 14 concurrent values on the positive side (with 0) and 15 on the negative (without). If SMALLINT were unsigned, you would have 15 with 0 and NULL. INT gives you 30, and BIGINT gives you 62. Essentially, that means that this is only a viable alternative if you need to assign fewer than 62 matches. Or, perhaps the NUMERIC data type can be used for bigger numbers?
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