but then what is the answer to his problem? - a way of searching all those DB in an easy manner?
Answer is NO. The search cannot span multiple databases.
It CAN span multiple collections though, and does so by default.
Something bothers me about the original post, that I didn't catch before. He says he's got hundreds of databases. He probably means collections. That's ok. Dozens of collections is fine. Hundreds is ok, he's got thousands of clips, and has a reasonable hierarchy that is meaningful. Hundreds of databases is NOT OK. Most users should only need one, some need two or three, but after that, you're just using a lot of overhead.