| From: | Alexander Korotkov <aekorotkov(at)gmail(dot)com> |
|---|---|
| To: | Andrei Lepikhov <a(dot)lepikhov(at)postgrespro(dot)ru> |
| Cc: | Alexander Lakhin <exclusion(at)gmail(dot)com>, Richard Guo <guofenglinux(at)gmail(dot)com>, "Gregory Stark (as CFM)" <stark(dot)cfm(at)gmail(dot)com>, Michał Kłeczek <michal(at)kleczek(dot)org>, PostgreSQL Hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org> |
| Subject: | Re: Removing unneeded self joins |
| Date: | 2024-05-02 10:07:54 |
| Message-ID: | CAPpHfdvGVcud2i00pEVH2_GS09c8UsDh6P-SEOLd6YMGOxiNEw@mail.gmail.com |
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| Lists: | pgsql-hackers |
On Thu, May 2, 2024 at 12:45 PM Andrei Lepikhov
<a(dot)lepikhov(at)postgrespro(dot)ru> wrote:
>
> On 5/1/24 18:59, Alexander Korotkov wrote:
> > I think we probably could forbid SJE for the tables with TABLESAMPLE
> > altogether. Please, check the attached patch.
> Your patch looks good to me. I added some comments and test case into
> the join.sql.
Thank you
> One question for me is: Do we anticipate other lateral self-references
> except the TABLESAMPLE case? Looking into the extract_lateral_references
> implementation, I see the only RTE_SUBQUERY case to be afraid of. But we
> pull up subqueries before extracting lateral references. So, if we have
> a reference to a subquery, it means we will not flatten this subquery
> and don't execute SJE. Do we need more code, as you have written in the
> first patch?
I think my first patch was crap anyway. Your explanation seems
reasonable to me. I'm not sure this requires any more code. Probably
it would be enough to add more comments about this.
------
Regards,
Alexander Korotkov
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