Datomic supports running a query across multiple databases, which is powerful -- [see this](http://hyperfiddle.net/dustingetz.com/ezpkb21haW4gbmlsLCA6cHJvamVjdCAiZHVzdGluZ2V0ei5jb20iLCA6bGluay1kYmlkICNEYklkWzE3NTkyMTg2MDQ1OTA5IDE3NTkyMTg2MDQ1NDIyXSwgOnF1ZXJ5LXBhcmFtcyB7OmVudGl0eSAjRGJJZFsxNzU5MjE4NjA0NTY1MyAxNzU5MjE4NjA0NTkwM119fQ,,) for an example.
Datomic supports running a query across multiple databases, which is powerful -- see this for an example.