Query hints should be used with caution, and a special care should be given to locking hints; in Books Online this subject is fully covered, and several SQL professionals out there have added their own views and experience, which is also very evident online. I've also used some examples of how query hints work when discussing optimistic concurrency (and some other places) – purely for illustration purposes.

Too often are locking hints missused, or at least used inappropriately – either when developers ignore the possibility of negative consequences altogether or when they focus purely on performance, rather than balancing the latter with data consistency and accuracy. Visit any SQL Server related newsgroup and you'll find really good examples of really bad query hint abuse. Luckily, these users (still) represent only a less significant minority. But on the other hand, when sensitive data is concerned a single member of this minority with their *special performance imporvements* is enough to cause mayhem. Joe Celko might also say: murder.

Tony Rogerson posted a very interesting article on his blog today dealing with the possibility of truly unpleasant consequences when the NOLOCK hint is used. Take a look at Tony's example, reproduce it on your system, and see for yourself how devastating such possibilities really can be. Great article!

Don't get me wrong, I'm not saying (and I don't think Tony is) that the NOLOCK hint should not be used at all – far from it: sometimes it really is the only way to go, but never should it be used without first considering the alternatives (if any) and possible consequences. In fact, if you ask me, the same goes for any query hint. Test, re-test, double-test, and – just to make sure – test again.

Further more, Tony has also announced more articles on the subject of concurrency and consistency, so keep an eye on his blog. I know I will.