We have a prod system that sometimes gets stuck and nothing we do helps but changing maxdop parameter on live system, after we change that all gets back normal. And it does not even matter if we change maxdop to 0 from 4, or to 2 from 1 , as long as we change maxdop. It has been going on for about a year (and during that time this has happened about 4 or 5 times) and we have not been able to find what is causing it. Since the system is going to be replaced soon and we have sort of workaround for it, I am not asking for help to figure this out, just to understand what happens to existing sessions when maxdop is changed, so my question is: What happens to queries that are in progress when we change maxdop in live system and while they are executing?