| From: | Robert Haas <robertmhaas(at)gmail(dot)com> |
|---|---|
| To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
| Cc: | David Rowley <david(dot)rowley(at)2ndquadrant(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: | 2019-03-22 16:16:54 |
| Message-ID: | CA+TgmobMD22W79FQL3a2ZJ+EpEb-W3L7inuc-4k-8gM2XinN3A@mail.gmail.com |
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| Lists: | pgsql-hackers |
On Fri, Mar 22, 2019 at 11:56 AM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> In cases where, say, the first child requires no sort but also doesn't
> emit very many rows, while the second child requires an expensive sort,
> the planner will have a ridiculously optimistic opinion of the cost of
> fetching slightly more rows than are available from the first child.
> This might lead it to wrongly choose a merge join over a hash for example.
I think this is very much a valid point, especially in view of the
fact that we already choose supposedly fast-start plans too often. I
don't know whether it's a death sentence for this patch, but it should
at least make us stop and think hard.
--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company
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