On Wed, Dec 6, 2017 at 10:20 PM, Craig Ringer <craig@2ndquadrant.com> wrote:
> Personally I think it's a pity we didn't land up here before the foundations
> for parallel query went in - DSM, shm_mq, DSA, etc. I know the EDB folks at
> least looked into it though, and presumably there were good reasons to go in
> this direction. Maybe that was just "community will never accept threaded
> conversion" at the time, though.
Yep. Never is a long time, but it took 3 release cycles to get a
user-visible feature as it was, and if I'd tried to insist on a
process->thread conversion first I suspect we'd still be stuck on that
point today. Perhaps we would have gotten as far as getting that much
done, but that wouldn't make parallel query be done on top of it.
> Now we have quite a lot of homebrew infrastructure to consider if we do a
> conversion.
>
> That said, it might in some ways make it easier. shm_mq, for example, would
> likely convert to a threaded backend with minimal changes to callers, and
> probably only limited changes to shm_mq its self. So maybe these
> abstractions will prove to have been a win in some ways. Except DSA, and
> even then it could serve as a transitional API...
Yeah, I don't feel too bad about what we've built. Even if it
ultimately goes away, it will have served the useful purpose of
proving that parallel query is a good idea and can work. Besides,
shm_mq is just a ring buffer for messages; that's not automatically
something that we don't want just because we move to threads. If it
goes away, which I think not unlikely, it'll be because something else
is faster.
Also, it's not as if only parallel query structures might have been
designed differently if we had been using threads all along.
dynahash, for example, is quite unlike most concurrent hash tables and
a big part of the reason is that it has to cope with being situated in
a fixed-size chunk of shared memory. More generally, the whole reason
there's no cheap, straightforward palloc_shared() is the result of the
current design, and it seems very unlikely we wouldn't have that quite
apart from parallel query. Install pg_stat_statements without a
server restart? Yes, please.
--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company