Re: Using left joins instead of inner joins as an optimization
| От | Greg Sabino Mullane |
|---|---|
| Тема | Re: Using left joins instead of inner joins as an optimization |
| Дата | |
| Msg-id | CAKAnmmLR0cGVWvkqXbPXu+ZAq0Vye=36VsnY5Qniy-gH4pZR1g@mail.gmail.com обсуждение исходный текст |
| Ответ на | Using left joins instead of inner joins as an optimization (Xavier Solomon <xavier.solomon515@gmail.com>) |
| Список | pgsql-general |
On Fri, Sep 6, 2024 at 7:05 AM Xavier Solomon <xavier.solomon515@gmail.com> wrote:
> explain select b_id from b natural left join a;
results in a `Seq Scan on b`. Whereas the query
> explain select b_id from b natural join a;
results in a join with sequential scans on both a and b.
I think your example is a little too contrived. Try explaining
select * from b natural left join a;
and you should see the plans become equivalent again.
I would expect a query that left joins but only pulls data from one table to be not feasible in real life. Yes, in an ideal world the non-left join would be smart enough to not even do the scan on a, but it's kind of a moot point outside of odd select clauses.
- Is it a bad idea to use left joins to optimize this even if semantically an inner join would be correct?
Not at all - if it works for you, go ahead. But I'm dubious you will gain much for queries that actually make use of the left join, at least for relatively simply selects.
Cheers,
Greg
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