| From: | Justin Pryzby <pryzby(at)telsasoft(dot)com> |
|---|---|
| To: | Richard Guo <guofenglinux(at)gmail(dot)com> |
| Cc: | Robert Haas <robertmhaas(at)gmail(dot)com>, bucoo(at)sohu(dot)com, pgsql-hackers(at)postgresql(dot)org, Tomas Vondra <tomas(dot)vondra(at)postgresql(dot)org>, Dmitry Dolgov <9erthalion6(at)gmail(dot)com> |
| Subject: | Re: fix cost subqueryscan wrong parallel cost |
| Date: | 2022-04-15 10:10:59 |
| Message-ID: | 20220415101059.GW26620@telsasoft.com |
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| Lists: | pgsql-hackers |
On Fri, Apr 15, 2022 at 05:16:44PM +0800, Richard Guo wrote:
> Not related to this topic but I noticed another problem from the plan.
> Note the first Sort node which is to unique-ify the result of the UNION.
> Why cannot we re-arrange the sort keys from (a, b, c) to (a, c, b) so
> that we can avoid the second Sort node?
I don't know, but it's possible there's a solution related to commit db0d67db2
"Optimize order of GROUP BY keys" - DISTINCT is the same as GROUP BY k1, ...,
kN. I guess UNION [DISTINCT] should learn to use GROUP BY rather than
DISTINCT?
--
Justin
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