| From: | Richard Guo <guofenglinux(at)gmail(dot)com> |
|---|---|
| To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
| Cc: | Robert Haas <robertmhaas(at)gmail(dot)com>, Alexandra Wang <alexandra(dot)wang(dot)oss(at)gmail(dot)com>, PostgreSQL Hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org>, "bruce(at)momjian(dot)us" <bruce(at)momjian(dot)us>, lepihov(at)gmail(dot)com |
| Subject: | Re: plan shape work |
| Date: | 2025-09-13 08:46:09 |
| Message-ID: | CAMbWs4_bEtEwePqX-akakr616=Ex4tCRafYtTQ4UZ4b6qmkwFg@mail.gmail.com |
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On Sat, Sep 13, 2025 at 12:08 AM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> After thinking about this for awhile, I believe that Richard and I
> each had half of the right solution ;-). Let me propose some new
> terminology in hopes of clarifying matters:
>
> * A join plan node "starts" an outer join if it performs the
> null-extension step corresponding to that OJ (specifically,
> if it is the first join doing null-extension over the minimum
> RHS of that OJ).
>
> * A join plan node "completes" an outer join if its output
> nulls all the values that that OJ should null when done
> according to syntactic order.
This new notion makes a lot of sense to me. I feel that it could help
us optimize some existing logic, or at least make certain parts of it
easier to understand. It might be worth adding to the README, maybe
under the section "Relation Identification and Qual Clause Placement",
where we explain the idea behind pushed-down joins.
- Richard
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