Re: Issues with ON CONFLICT UPDATE and REINDEX CONCURRENTLY

Hello, Álvaro!

On Thu, Dec 11, 2025 at 10:36 PM Álvaro Herrera <alvherre(at)kurilemu(dot)de> wrote:
> I just saw a failure in CI for an unrelated patch

I'll try to dive deeper tomorrow to find a fix, but it feels like we
are doing something wrong here.

The tests were good to prove the issue and demonstrate it was fixed
after some changes.

But currently we are just trying (not the first time already) to make
sure OUTPUT of the test is EXACTLY equal to some variant.
At the same time I think a more correct approach here - is to test
something like "output does not contain `duplicate key value violates
unique constraint` message". Or even better real case - pgbench of
concurrent REINDEX + INSERT (takes seconds to reproduce, but CPU is
high).
It is a way to test something essential what we want to be not broken,
not exact output of concurrent commands.... But current
isolationtester does not support anything like that.

I am afraid amount of time needed to stabilize such test (in its
output, not the sense) is not cover potential value of it.
Also, I imaging someone changing something unrelated (catalog snapshot
invalidation, for example) and test starts to fail on some rear animal
once a week.... Ughn.

Maybe I am inclined by my main programming experience (Java, backends,
distributed systems, etc.) and databases need to be much more accurate
and strict even if it pains...

What do you think about it?

Best regards,
Mikhail.