Re: Planner : anti-join on left joins

Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> 于2026年1月1日周四 07:37写道：

> Nicolas Adenis-Lamarre <nicolas(dot)adenis(dot)lamarre(at)gmail(dot)com> writes:
> > - detect anti join on "a left join b where x is null" where x is a non
> null
> > var b (b being a rte)
> > this is the object of the attached patched.
>
> This is a perfectly reasonable thing to do, especially now that we've
> built out some infrastructure that would help. It doesn't look like
> your patch is using that though. Take a look at commits 904f6a593
> and e2debb643.
>

Yes, after commits 904f6a593 and e2debb643, we have some infrastructure to
use.
I provided a patch to implement this reduction using these infrastructure
codes.
Please check the attached patch.

>
> BTW, it is not a good look for even a draft patch to not bother
> updating adjacent comments that it falsifies, such as this in
> reduce_outer_joins_pass2:
>
> * See if we can reduce JOIN_LEFT to JOIN_ANTI. This is the case
> if
> * the join's own quals are strict for any var that was forced
> null by
> * higher qual levels. NOTE: there are other ways that we could
> * detect an anti-join, in particular if we were to check whether
> Vars
> * coming from the RHS must be non-null because of table
> constraints.
> * That seems complicated and expensive though (in particular, one
> * would have to be wary of lower outer joins). For the moment this
> * seems sufficient.
>
> In the long run, the comments are as important as the code, if not
> even more so. Keeping them accurate is not optional.
>

I updated the comments in the attached patch as well.
And I test the regression in the src, all tests pass.
Any thoughts?

--
Thanks,
Tender Wang