Yeah, it’s definitely more expensive (and abruptly so!)
About the more compact table format: Sean Corfield mentioned that in the next version of clojure.java.jdbc he does not convert to Clojure data structures at all, but instead he implemented a reducible sequence over the ResultSet instance. That seems like a good way to keep the convenience of working with the regular core functions when wrapping external libs. If they’re all reducible, it won’t matter what the underlying implementation is (and we can probably swap them transparently).