From: Tom Lane <tgl@sss.pgh.pa.us>
To: David Rowley <dgrowleyml@gmail.com>
cc: Maxim Boguk <maxim.boguk@gmail.com>, pgsql-bugs@lists.postgresql.org
Subject: Re: BUG #18588: Cannot force/let database use parallel execution in
 simple case.
In-reply-to: 
 <CAApHDvqVvFz1L-Yxyts+Q4+rbyuOmsJwFK9=pJwQS=cukb+z0g@mail.gmail.com>
References: <18588-204730f9394441a8@postgresql.org>
 <CAApHDvqGgO7c8N0kq5H4jZ0NZe=NGZDo4VVF5Fi5CVdJNbvStg@mail.gmail.com>
 <CAK-MWwTqDRerxrpaN8Xb25DqDGBw1UT+YSkfp4QN-MgOH97c+Q@mail.gmail.com>
 <CAK-MWwS_ZAsaGrz0byqCO38NNrNqPkNt1ww1dorHz_wiaFbWvw@mail.gmail.com>
 <2059550.1724353316@sss.pgh.pa.us> <2073208.1724359323@sss.pgh.pa.us>
 <CAApHDvqVvFz1L-Yxyts+Q4+rbyuOmsJwFK9=pJwQS=cukb+z0g@mail.gmail.com>
Comments: In-reply-to David Rowley <dgrowleyml@gmail.com>
	message dated "Fri, 23 Aug 2024 10:44:27 +1200"
MIME-Version: 1.0
Content-Type: text/plain; charset="us-ascii"
Content-ID: <2088497.1724367160.1@sss.pgh.pa.us>
Date: Thu, 22 Aug 2024 18:52:40 -0400
Message-ID: <2088498.1724367160@sss.pgh.pa.us>
Archived-At: 
 <https://www.postgresql.org/message-id/2088498.1724367160%40sss.pgh.pa.us>
Precedence: bulk

David Rowley <dgrowleyml@gmail.com> writes:
> I played around with the attached script and set some breakpoints in
> cost_index(). I'm seeing the same thing as you with the parallel path
> being only slightly cheaper, but when looking at cost_index(), it's
> easy to see why.

> It's only the cpu_run_cost that's divided by the parallel_divisor.

D'oh.

What Maxim is observing is that he's getting I/O concurrency from
multiple workers; and that's an effect we're simply not accounting
for here.  cost_seqscan excuses the same omission with

         * It may be possible to amortize some of the I/O cost, but probably
         * not very much, because most operating systems already do aggressive
         * prefetching.  For now, we assume that the disk run cost can't be
         * amortized at all.

but I don't think this prefetch argument applies to index scans.

			regards, tom lane