Seems that [data-warehouse] and [database-design] questions are being migrated from StackOverflow by (?) to Database Administrators -- here is an example.
Is this now an established policy?
Is there a rule when a question is being migrated?
Speaking from the standpoint of someone "in the know" over at [dba.se], I would encourage you to first check the FAQ (which to my own fault is lacking in replete defintion, but we've recently held mod elections, so we should be sprucing that up .. soonish) but then to follow it up with a brief visit to our meta, wherein the FAQ tab of the Questions page on that meta (which is the canonical-to-date place to find such questions, as it would be here on Meta.Stack Overflow) shows this Q:
We see what questions are encouraged to be asked there.
However, what you're really wanting to know is: We've been asking for selected questions to be migrated, because we wanted to build a better core set of questions of the variety that we wanted to see, and we haven't asked for questions to be migrated en-masse because cherry picking gives us what we want and keeps things clean.
So, does this mean "yay, migrate all the things"? No, not at all. But if you're curious about if your question would be better suited on a site dedicated to getting help from senior DBAs (whether they be NoSQL or RDBMS guys) and you can't tell from browsing the site first (hey, your question could already be well answered!) then feel free to ask on http://meta.dba.stackexchange.com or pop into the chatroom:
HTH (but it won't have helped at all, I'm sure)
AFAIK there is no rule or policy on this, it boils down to what the SO community wants and whether there are questions they would like to send to a better home if there was one - but over on dba.se our intention is to provide a good home for:
Looking through some of your answers, I think you'd be a valuable contributor on dba.se should you choose to share yourself between us and SO - personally, I hope you will consider doing so, and ultimately I think you would be very happy on the site as it grows and becomes better known