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

Robert Haas <robertmhaas@gmail.com> writes:
> On Thu, Apr 2, 2015 at 2:40 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> However, I'm having second thoughts about whether we've fully diagnosed
>> this.  Three out of the four failures we've seen in the buildfarm reported
>> "cache lookup failed for access method 403", not "could not open relation
>> with OID 2601" ... and I'm so far only able to replicate the latter
>> symptom.  It's really unclear how the former one could arise, because
>> nothing that vacuum.sql does would change xmin of the rows in pg_am.

> It probably changes the *relfilenode* of pg_am, because it runs VACUUM
> FULL on that catalog.  Perhaps some backend sees the old relfilenode
> value and tries to a heap scan, interpreting the now-truncated file as
> empty?

Yeah, I came up with the same theory a few minutes later.  Trying to
reproduce on that basis.

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.
        regards, tom lane