Horton

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

Tom Lane <tgl@sss.pgh.pa.us>

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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Arseniy Mukhin <arseniy.mukhin.dev@gmail.com>
Cc: washwithcare@gmail.com, pgsql-bugs@lists.postgresql.org, Nikita Glukhov <glukhov.n.a@gmail.com>, Alexander Korotkov <aekorotkov@gmail.com>
Date: 2025-08-27T14:32:26Z
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.

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.

			regards, tom lane