Oracle and SQL Server don't have a community. Please understand when I say that, I mean that the chance that you will get to talk to a developer of the core database is about the same as your chance of winning the lottery. By the time you do, it's probably because you found a bug so heinous that it couldn't be ignored and the only person who can understand your report is the guy who wrote the code in question. They have paid technical support and this support has proven in my experience to be generally competent, but not stellar. I have had to work around the problem that I originally requested help with about 40 percent of the time.
Compare this to MySQL and PostgreSQL, where just about anybody can speak to just about anybody else all day long. Many of the core developers of both the platforms can be found on IRC, met at conventions, contacted for contract development work, and for the most part, bribed remarkably easily with beer (hint, hint, wink, wink, nudge, nudge).
They are actively concerned about the health of the overall community and will answer just about any kind of question you ask, even if the question has a very tenuous relationship to database development. My personal experience, is that the PostgreSQL team has more core developers readily available than MySQL. They are also more personally available at conventions and meetings.