Re: Streaming read-ready sequential scan code

On Wed, Aug 28, 2024 at 9:00 PM Alexander Lakhin <exclusion@gmail.com> wrote:
> 2024-08-28 08:41:59.304 UTC [338251] DEBUG:  starting background worker process "parallel worker for PID 338262"
> 2024-08-28 08:41:59.304 UTC [338251] DEBUG:  starting background worker process "parallel worker for PID 338262"

> 2024-08-28 08:41:59.305 UTC [338263] DEBUG:  InitPostgres
> 2024-08-28 08:41:59.305 UTC [338264] DEBUG:  InitPostgres

> 2024-08-28 08:41:59.309 UTC [338263] NOTICE:  2024-08-28 08:41:59.309364+00 pid 338263
> 2024-08-28 08:41:59.310 UTC [338264] NOTICE:  2024-08-28 08:41:59.310079+00 pid 338264

> It looks like the two parallel workers were started simultaneously, but
> then the second one lagged behind...

Yeah.  That's quite interesting, and must destabilise that
simple-minded demo.  I'm curious to know exactly what contention is
causing that (about 3/4 of a millisecond that I don't see and now I
want to know what it's waiting for), but it's a very crude test
lacking timer resolution in the earlier messages, and it's an
unrelated topic and a distraction.  Perhaps it explains why you saw
two different behaviours in Q15 with the patch and I didn't, though.
Really it shouldn't be so sensitive to such variations, it's obviously
a terrible plan, and TPC-DS needs a planner hacker mega-brain applied
to it; I'm going to try to nerd-snipe one...