Re: RI fast path gets cross-type foreign keys wrong

On Fri, Aug 7, 2026 at 10:16 PM Amit Langote <amitlangote09(at)gmail(dot)com> wrote:
> On Thu, Aug 6, 2026 at 11:11 PM Amit Langote <amitlangote09(at)gmail(dot)com> wrote:
> > On Sun, Aug 2, 2026 at 12:59 AM Amit Langote <amitlangote09(at)gmail(dot)com> wrote:
> > > On Sat, Aug 1, 2026 at 8:49 Amit Langote <amitlangote09(at)gmail(dot)com> wrote:
> > >> On Sat, Aug 1, 2026 at 8:33 Peter Geoghegan <pg(at)bowt(dot)ie> wrote:
> > >>>
> > >>> The attached isolation test patch (written by Claude code) shows that
> > >>> the new RI fast path can get some things wrong when a foreign key uses
> > >>> a cross-type equality operator.
> > >>>
> > >>> The test has two permutations differing only in the primary key's type
> > >>> -- date against a timestamp FK in the first, timestamp against
> > >>> timestamp in the second. The interleaving is identical and the
> > >>> referenced key is present throughout, so both should behave the same
> > >>> way. The second succeeds (which is correct), while the first fails
> > >>> with a foreign key violation (which is incorrect).
> > >>>
> > >>> The fast path was introduced by b7b27eb41. This is an issue on 19 and
> > >>> master only.
> > >>
> > >>
> > >> Thanks Peter for the report. I’m on vacation atm, will take a look when I get back next week.
> > >
> > > Still away, but couldn’t help reading the code on my phone.
> > >
> > > The recheck block in ri_FastPathFlushArray() appears to put the key it has just read out of the locked tuple (found_val) into sk_argument, and then pass that same slot to recheck_matched_pk_tuple(). If I’m reading that right, both operands come from the locked tuple, so it compares the key with itself and since sk_argument is the right-hand operand, which on a cross-type operator is the FK-typed side, the PK value there gets read as an FK value. That would make the same-type case pass trivially and the cross-type case fail always, which seems to fit what Peter is seeing.
> > >
> > > If that’s really what’s going on, maybe the recheck can just go away, because the loop a few lines further down in the same scan iteration already compares found_val against the buffered FK values. I’ll look at it properly next week.
> >
> > The reading on the phone was right, so removing the recheck works and
> > is fine. The marking loop a few lines below already compares
> > found_val, read after the chain has been followed, against every
> > buffered FK value, with the arguments in the order the operator
> > expects, so a key that has moved away matches nothing and we still
> > report the violation. Reorienting the scan key isn't an option, since
> > sk_argument would have to be the particular buffered value that this
> > tuple matched, and the AM doesn't expose which array element that was.
> >
> > ri_FastPathProbeOne() passes its original scan key, with the FK value
> > still in sk_argument, so it was never affected. Single-row statements
> > go through it, and Peter's test passes on unpatched HEAD if the INSERT
> > adds one row instead of two.
> >
> > Attached is a patch. I kept Peter's test and added a permutation that
> > leaves the key where it was moved, so the check must report a
> > violation.
>
> Would like to commit tomorrow, barring objections.

Pushed (cc449a504bf) and marked open item for this issue as fixed.

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Thanks, Amit Langote