I've pushed (and backpatched) a fix for this. I ended up doing the simplest thing -- error out if the number of columns does not match, suggesting to update to latest extension version.
I considered handling it in a nicer way, but I didn't like the result very much and I think that's sufficient for superuser-only extension. And 691e8b2e18 seems like a reasonable precedent (even though the backbranches did do a different thing). I also considered introducing pg_stat_statements-style versioning, but it's too late to do that in backbranches, and I don't think we expect the function to change very often to justify this. regards -- Tomas Vondra
