Re: Something is rotten in the state of Denmark...

Robert Haas <robertmhaas@gmail.com> writes:
> On Thu, Apr 2, 2015 at 2:55 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> Actually, now that I think it through, the "could not open relation"
>> error is pretty odd in itself.  If we are trying to open pg_am using
>> a stale catalog snapshot, it seems like we ought to reliably find its
>> old pg_class tuple (the one with the obsolete relfilenode), rather than
>> finding nothing.  But the latter is the behavior I'm seeing.

> What's to stop the old tuple from being HOT-pruned?

Hm, that may be it.  I went back to the previous test scenario, and now
I can *only* get the "cache lookup failed for access method" behavior,
instead of what I was getting before, so I'm getting a bit confused :-(.
However, it does seem clear that the mechanism is indeed that we're
relying on an obsolete copy of pg_am's pg_class tuple, hence scanning a
truncated relfilenode, and that the patch I proposed fixes it.

Perhaps the difference has to do with whether pg_am's pg_class tuple is
on a page that hasn't got enough room for a HOT update?  But I definitely
tried it several times and consistently got the same failure before.
        regards, tom lane