| From: | Andrei Lepikhov <lepihov(at)gmail(dot)com> |
|---|---|
| To: | Ashutosh Bapat <ashutosh(dot)bapat(dot)oss(at)gmail(dot)com> |
| Cc: | Jakub Wartak <jakub(dot)wartak(at)enterprisedb(dot)com>, Tomas Vondra <tomas(dot)vondra(at)enterprisedb(dot)com>, arne(dot)roland(at)malkut(dot)net, PostgreSQL Hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Julien Rouhaud <rjuju123(at)gmail(dot)com>, Etsuro Fujita <etsuro(dot)fujita(at)gmail(dot)com>, Richard Guo <guofenglinux(at)gmail(dot)com> |
| Subject: | Re: apply_scanjoin_target_to_paths and partitionwise join |
| Date: | 2024-09-30 21:52:11 |
| Message-ID: | c55907c5-8b1d-42a5-8bb5-b4c9d92c993f@gmail.com |
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| Lists: | pgsql-hackers |
On 24/7/2024 15:22, Ashutosh Bapat wrote:
> On Wed, Jul 24, 2024 at 9:42 AM Richard Guo <guofenglinux(at)gmail(dot)com> wrote:
>> Is there a specific query that demonstrates benefits from this change?
>> I'm curious about scenarios where a partitionwise join runs slower
>> than a non-partitionwise join.
>
> [1] provides a testcase where a nonpartitionwise join is better than
> partitionwise join. This testcase is derived from a bug reported by an
> EDB customer. [2] is another bug report on psql-bugs.
I haven't passed through the patch yet, but can this issue affect the
decision on what to push down to foreign servers: a whole join or just a
scan of two partitions?
If the patch is related to the pushdown decision, I'd say it is quite an
annoying problem for me. From time to time, I see cases where JOIN
produces more tuples than both partitions have in total - in this case,
it would be better to transfer tables' tuples to the main instance
before joining them.
--
regards, Andrei Lepikhov
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