From: Tom Lane <tgl@sss.pgh.pa.us>
To: Robert Haas <robertmhaas@gmail.com>
cc: James Coleman <jtc331@gmail.com>,
        pgsql-hackers <pgsql-hackers@postgresql.org>,
        Amit Kapila <amit.kapila16@gmail.com>
Subject: Re: Parallelize correlated subqueries that execute within each worker
In-reply-to: <CA+TgmoYXm2NCLt1nikWfYj1_r3=fsoNCHCtDVdN7X1uX_xuXgw@mail.gmail.com>
References: <CAAaqYe_x1u3V4uPiv=dJ=k2EJ7txhdq6yexJDkYZ1x1pu0QwcQ@mail.gmail.com> <CA+TgmobtK2OnhNaKbq8Q+k217mCUdpLFeMXTjxr6QZMG3KE5Gw@mail.gmail.com> <CAAaqYe-FYVq=CSZu8VtKi0fx8Zdi6fzyzEP2B9iK2cUgY8xGjA@mail.gmail.com> <CA+TgmoaAPfPcQx0uYLusU4A+Vm5Cr2F8irxo=xf8BQ-97YyZ7A@mail.gmail.com> <CAAaqYe8HLwTGJC2jPZ8noiv2SRSSvS3zSjQQMmeUqNiY4Uw_dw@mail.gmail.com> <CA+TgmoYXm2NCLt1nikWfYj1_r3=fsoNCHCtDVdN7X1uX_xuXgw@mail.gmail.com>
Comments: In-reply-to Robert Haas <robertmhaas@gmail.com>
	message dated "Fri, 21 Jan 2022 15:20:44 -0500"
MIME-Version: 1.0
Content-Type: text/plain; charset="us-ascii"
Content-ID: <1279418.1642798443.1@sss.pgh.pa.us>
Date: Fri, 21 Jan 2022 15:54:03 -0500
Message-ID: <1279419.1642798443@sss.pgh.pa.us>
Archived-At: <https://www.postgresql.org/message-id/1279419.1642798443%40sss.pgh.pa.us>
Precedence: bulk

Robert Haas <robertmhaas@gmail.com> writes:
> I don't think there's an intrinsic problem with the idea of making a
> tentative determination about parallel safety and then refining it
> later, but I'm not sure why you think it would be a lot of work to
> figure this out at the point where we generate gather paths. I think
> it's just a matter of testing whether the set of parameters that the
> path needs as input is the empty set. It may be that neither extParam
> nor allParam are precisely that thing, but I think both are very
> close, and it seems to me that there's no theoretical reason why we
> can't know for every path the set of inputs that it requires "from the
> outside."

I'd be very happy if someone redesigned the extParam/allParam mechanism,
or at least documented it better.  It's confusing and I've never been
able to escape the feeling that it's somewhat redundant.

The real problem with it though is that we don't compute those values
until much too late to be useful in path construction; see comments
for SS_identify_outer_params.  To be helpful to the planner, we'd have
to rejigger things at least enough to calculate them earlier -- or
maybe better, calculate what the planner wants earlier, and then transform
to what the executor wants later.

			regards, tom lane