I've known about the Query Store for some time, however, I've been bound by an NDA that prevented me from talking about it. However this week I saw a piece on SQL Performance that described some details about the feature and how it should work. Essentially the Query Store (QS) is a monitoring, metrics, and data store for execution plans used in queries. I'd urge you to read the article if you're interested, but essentially the QS captures the execution plans when a plan is compiled, and some statistics when the plan is executed.

The idea here is to give us more insight, and knowledge, about how the SQL Server database engine performs with our workloads. Many of us have been stumped at times as to why performance has suddenly changed, and while the DMVs available in SQL Server can help you troubleshoot, they suffer from a lack of permanancy across reboots, and also a limited amount of history. They're fairly complex to dig into and correlate information, and the QS is designed to do this for you, making it easier to understand when performance has changed. Not necessarily why, but at least you know when the server's performance has gone south.

I'm excited for this feature, mostly because it opens up a whole new range of analysis that we can perform on our servers. Potenially we'll even see people working with workloads, tracking exactly when a row update or selectivity alteration causes the query optimizer to choose a new plan. While Microsoft devotes a lot of work to building the best query processor they can, they don't have unlimited resources. My hope is that many people in the community will delve deeply into the data in the QS and feedback their analysis and conclusions to Microsoft, who may then find ways to improve performance further.

Of course, I expect many people will just go to the QS when they have an issue and look to force a previous plan to be used by the engine. That's fine, too, and it's what this feature helps you do. That might not solve your problem, but like a reboot, it is probably a good place to start when you don't understand the problem.