Merlin Moncure <mmoncure@xxxxxxxxx> writes: > On Wed, Oct 30, 2019 at 5:41 PM Andres Freund <andres@xxxxxxxxxxx> wrote: >> Hm. Wouldn't it be fairly easy to allow the client to specify how much >> metadata they'd want? I.e. opt-in into getting more complete metadata? > Suppose you had a set of 1000 records, with an array of composites (10 > items per array). How many times would the extra metadata describing > get sent following this approach? Presumably once per Describe request (or once per simple Query) ... but that facile answer actually isn't very satisfactory. It's at least theoretically possible that a column of type "record" could have varying actual rowtype from one tuple to the next. I don't want to swear that every part of Postgres is capable of coping with that, but a lot is --- see record_out() itself for the prototypical coding pattern. The wire protocol is really pretty tightly bound to the assumption that every tuple in a resultset has the same description, but that's going to fall down if we want to transmit details about what "record" means. The same problem occurs with arrays, specifically anyarray --- and here I don't have to think hard to come up with a query that will break it. "select * from pg_stats" is enough to do that. It's not only the wire protocol that's going to have issues with varying rowtypes. How would we deal with that in libpq's resultset API, for example? I wonder whether the JDBC API could handle it either. tl;dr: there are a lot more worms in this can than you might guess. regards, tom lane
