| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | Stefan Fehrenbach <stefan(dot)fehrenbach(at)gmail(dot)com> |
| Cc: | pgsql-performance(at)lists(dot)postgresql(dot)org |
| Subject: | Re: When can joins be avoided? |
| Date: | 2022-11-17 14:49:20 |
| Message-ID: | 1826810.1668696560@sss.pgh.pa.us |
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| Lists: | pgsql-performance |
Stefan Fehrenbach <stefan(dot)fehrenbach(at)gmail(dot)com> writes:
> I'm interested in the rules that Postgres uses to avoid joins. Are
> these documented somewhere? If I had to look at the source code, where
> would I start?
src/backend/optimizer/plan/analyzejoins.c
> They don't seem to match my intuition about which joins
> could be avoided.
I believe only left joins to single tables can be elided ATM.
It's too hard to prove uniqueness of the join key in more-
complicated cases.
regards, tom lane
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