On the initial subscription, 'callback' would be called once for each row in 'tableA', then again when a row is inserted or updated. It would be the same as Firebase in this respect.
Not OP but I would watch for mutations in the subscribed columns, then test the changed rows on the query's constraints and fire an event if it satisfies them.
I'm sure there is a more clever/efficient way of doing this though, and I can't wait to see how he/she implements it.
