| From: | David Rowley <david(dot)rowley(at)2ndquadrant(dot)com> |
|---|---|
| To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
| Cc: | Robert Haas <robertmhaas(at)gmail(dot)com>, Julien Rouhaud <rjuju123(at)gmail(dot)com>, Antonin Houska <ah(at)cybertec(dot)at>, Amit Langote <Langote_Amit_f8(at)lab(dot)ntt(dot)co(dot)jp>, PostgreSQL Hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org> |
| Subject: | Re: Ordered Partitioned Table Scans |
| Date: | 2019-03-23 03:05:05 |
| Message-ID: | CAKJS1f93zOtkYMT46fGPF7sUsRDFqh=3He_2tVkorFtYBmcyig@mail.gmail.com |
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On Sat, 23 Mar 2019 at 05:40, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> BTW, another thing we could possibly do to answer this objection is to
> give the ordered-Append node an artificially pessimistic startup cost,
> such as the sum or the max of its children's startup costs. That's
> pretty ugly and unprincipled, but maybe it's better than not having the
> ability to generate the plan shape at all?
I admit to having thought of that while trying to get to sleep last
night, but I was too scared to even suggest it. It's pretty much how
MergeAppend would cost it anyway. I agree it's not pretty to lie
about the startup cost, but it does kinda seem silly to fall back on a
more expensive MergeAppend when we know fine well Append is cheaper.
Probably the danger would be that someone pulls it out thinking its a
bug. So we'd need to clearly comment why we're doing it.
--
David Rowley http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services
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