Обсуждение: assembling PGresults from multiple simultaneous queries (libpq, singlerowmode)

Hi,

On 2019-04-07 20:57:56 +0200, Pavel Stehule wrote:
> ne 7. 4. 2019 v 20:47 odesílatel Konstantin Izmailov <pgfizm@gmail.com>
> napsal:
>
> > Hi,
> > I'm experimenting with Postgres 10 and protocol v3. I noticed that the
> > Postgres allows executing multiple queries simultaneously (I basically
> > commented out a check that prevents sending another query in libpq while
> > previous result(s) reading is not complete). Things appear like working,
> > but I just wanted to ask if anyone else tried the same (logically separate
> > results from multiple simultaneous queries)?
> >
> >
> Postgres cannot to execute simultaneous queries in one session. So queries
> should be executed in serial form every time.

I think what Konstantin is really talking about is pipelining
(i.e. sending multiple queries without waiting for the results
inbetween, thereby reducing latency), and that is actually supported by
postgres. Some drivers make fairly extensive use of it (e.g. the pgjdbc
driver).

As for libpq: No, that's currently not supported. There is a patch that
I hope to get into v13 however: https://commitfest.postgresql.org/20/1317

Greetings,

Andres Freund

Yes, Andres, I meant "pipelining", just couldn't choose correct word. Thank you for the answer(s)!

I also made changes in my own copy of libpq, and they work fine. I think the pipelining support is needed in libpq. Btw, how can I get the patch code? I want to compare your approach with mine. I couldn't figure out how to get the patch from the link.

Thanks so much!

On Sun, Apr 7, 2019 at 12:21 PM Andres Freund <andres@anarazel.de> wrote:

Hi,

On 2019-04-07 20:57:56 +0200, Pavel Stehule wrote:
> ne 7. 4. 2019 v 20:47 odesílatel Konstantin Izmailov <pgfizm@gmail.com>
> napsal:
>
> > Hi,
> > I'm experimenting with Postgres 10 and protocol v3. I noticed that the
> > Postgres allows executing multiple queries simultaneously (I basically
> > commented out a check that prevents sending another query in libpq while
> > previous result(s) reading is not complete). Things appear like working,
> > but I just wanted to ask if anyone else tried the same (logically separate
> > results from multiple simultaneous queries)?
> >
> >
> Postgres cannot to execute simultaneous queries in one session. So queries
> should be executed in serial form every time.

I think what Konstantin is really talking about is pipelining
(i.e. sending multiple queries without waiting for the results
inbetween, thereby reducing latency), and that is actually supported by
postgres. Some drivers make fairly extensive use of it (e.g. the pgjdbc
driver).

As for libpq: No, that's currently not supported. There is a patch that
I hope to get into v13 however: https://commitfest.postgresql.org/20/1317

Greetings,

Andres Freund