Re: recovering from "found xmin ... from before relfrozenxid ..."

Robert Haas <robertmhaas@gmail.com> writes:
> On Sun, Sep 20, 2020 at 1:13 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> 2. As Amit suspected, there's an inconsistency between pruneheap.c's
>> rules for which tuples are removable and vacuum.c's rules for that.
>> This seems like a massive bug in its own right: what's the point of
>> pruneheap.c going to huge effort to decide whether it should keep a
>> tuple if vacuum will then kill it anyway?  I do not understand why
>> whoever put in the GlobalVisState stuff only applied it in pruneheap.c
>> and not VACUUM proper.

> I am not sure I fully understand why you're contrasting pruneheap.c
> with vacuum here, because vacuum just does HOT pruning to remove dead
> tuples - maybe calling the relevant functions with different
> arguments, but it doesn't have its own independent logic for that.

Right, but what we end up with is that the very same tuple xmin and
xmax might result in pruning/deletion, or not, depending on whether
it's part of a HOT chain or not.  That's at best pretty weird, and
at worst it means that corner-case bugs in other places are triggered
in only one of the two scenarios ... which is what we have here.

> The key point is that the freezing code isn't, or at least
> historically wasn't, very smart about dead tuples. For example, I
> think if you told it to freeze something that was dead it would just
> do it, which is obviously bad. And that's why Andres stuck those
> sanity checks in there. But it's still pretty fragile. I think perhaps
> the pruning code should be rewritten in such a way that it can be
> combined with the code that freezes and marks pages all-visible, so
> that there's not so much action at a distance, but such an endeavor is
> in itself pretty scary, and certainly not back-patchable.

Not sure.  The pruning code is trying to serve two masters, that is
both VACUUM and on-the-fly cleanup during ordinary queries.  If you
try to merge it with other tasks that VACUUM does, you're going to
have a mess for the second usage.  I fear there's going to be pretty
strong conservation of cruft either way.

FWIW, weakening the sanity checks in heap_prepare_freeze_tuple is
*not* my preferred fix here.  But it'll take some work in other
places to preserve them.

			regards, tom lane