Maybe I am missing something, but how can you possibly have an async SQL driver without threads like this? This sounds like a case of your Node.js database driver hiding the exact same behaviour described here within C code.
If the wire protocol for the driver is published, then you can write a 100% async driver for it. I.e. no threads blocking, ever. In fact, I already did this for redis and vert.x (I will dig out the code for this some time).
If you are dealing with something where you don't know what the wire protocol is and you just have a blocking client library to play with (e.g. JDBC - JDBC is, by definition blocking - see the JDBC API), then you can't do much but to wrap the blocking api in an async facade and limit the number of threads that block at any one time.
This is exactly what we do in vert.x. We accept the fact that many libraries in the Java world are blocking (e.g. JDBC) so we allow you to use them by running them on as a worker.
This is one area where we differ from node.js. Node.js makes you run everything on an event loop. This is just silly for some things, e.g. long running computations (remember the Fibonacci number affair?), or calling blocking apis.
With vert.x you run "event-loopy" things on the event loop but you can run "non event-loopy" things on a worker. It's a hybrid.
A limited number of threads will not scale as real async wake-on-data connections will scale. If demand is higher than your thread pool, for the use case that you're web response builds on async backend requests, your site will be down.
(notice the Connect method on line 325 of binding.cc)
At some level a client-server database driver isn't all that different from any other network client; you send a request over a socket and wait for a result. There's no reason you have to block while waiting.
Moreover some databases (like Postgres) let you receive asynchronous notifications signaled by transactions on other connections; that's how trigger-based replication systems like Bucardo do their thing.
Because it would be based on asynchronous socket responses. So you wouldn't iterate like you currently do w/ a ResultSet but rather have a simple "RowHandler" or sorts. However you still run into the trouble you do w/ node if you decide to do a lot of blocking work in there instead of just sending the row to some ExecutorService thread to get worked on.
