Horton

Re: BUG #19031: pg_trgm infinite loop on certain cases

Arseniy Mukhin <arseniy.mukhin.dev@gmail.com>

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From: Arseniy Mukhin <arseniy.mukhin.dev@gmail.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: washwithcare@gmail.com, pgsql-bugs@lists.postgresql.org, Nikita Glukhov <glukhov.n.a@gmail.com>, Alexander Korotkov <aekorotkov@gmail.com>
Date: 2025-08-27T19:17:24Z
Lists: pgsql-bugs

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  1. Put "excludeOnly" GIN scan keys at the end of the scankey array.

  2. Do CHECK_FOR_INTERRUPTS inside, not before, scanGetItem.

On Wed, Aug 27, 2025 at 5:32 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
>
> Arseniy Mukhin <arseniy.mukhin.dev@gmail.com> writes:
> > Good point, thanks for the explanation. I forgot that there can be
> > many attributes. And I agree, the more determinism in the system, the
> > easier it is to work with it and the less room for bugs. OTOH it seems
> > from the performance POV we want to have the stricter keys to be the
> > first so we do less work and fail fast on the first keys. It looks
> > like these two rules (excludeOnly keys LAST and more restrictive keys
> > FIRST) are kind of in conflict with each other. I tried to do some
> > experiments and it's seems GIN quite sensitive to it, at least in this
> > artificial example:
>
> Yeah, it is.  I recall seeing some comments to the effect that
> optimizing the order of scan keys would be a good thing, but if there
> is any code in there that tries to do so, I'm not seeing where.
> Seems like a fertile area for future research.
>
> > With applying patch both queries show the same time (second one). So
> > currently the user can tune the query by defining more restrictive
> > keys first. With the proposed fix it looks like users will have less
> > freedom here.
>
> I think most people would consider it a bug if they have to tune the
> order of the WHERE clauses manually.  The original statement of the
> current bug was basically that: it worked in one order and not the
> other.
>

Ok. I checked the patches. The bug is gone. Everything looks correct.

Thank you!

Best regards,
Arseniy Mukhin