Re: pg_amcheck contrib application

On Wed, Mar 31, 2021 at 12:34 AM Mark Dilger
<mark(dot)dilger(at)enterprisedb(dot)com> wrote:
> I'm not looking at the old VACUUM FULL code, but my assumption is that if the xvac code were resurrected, then when a tuple is moved off by a VACUUM FULL, the old tuple and associated toast cannot be pruned until concurrent transactions end. So, if amcheck is running more-or-less concurrently with the VACUUM FULL and has a snapshot xmin no newer than the xid of the VACUUM FULL's xid, it can check the toast associated with the moved off tuple after the VACUUM FULL commits. If instead the VACUUM FULL xid was older than amcheck's xmin, then the toast is in danger of being vacuumed away. So the logic in verify_heapam would need to change to think about this distinction. We don't have to concern ourselves about that, because VACUUM FULL cannot be running, and so the xid for it must be older than our xmin, and hence the toast is unconditionally not safe to check.
>
> I'm changing the comments back to how you had them, but I'd like to know why my reasoning is wrong.

Let's start by figuring out *whether* your reasoning is wrong. My
assumption was that old-style VACUUM FULL would move tuples without
retoasting. That is, if we decided to move a tuple from page 2 of the
main table to page 1, we would just write the tuple into page 1,
marking it moved-in, and on page 2 we would mark it moved-off. And
that we would not examine or follow any TOAST pointers at all, so
whatever TOAST entries existed would be shared between the two tuples.
One tuple or the other would eventually die, depending on whether xvac
went on to commit or abort, but either way the TOAST doesn't need
updating because there's always exactly 1 remaining tuple using
pointers to those TOAST values.

Your assumption seems to be the opposite, that the TOASTed values
would be retoasted as part of VF. If that is true, then your idea is
right.

Do you agree with this analysis? If so, we can check the code and find
out which way it actually worked.

--
Robert Haas
EDB: http://www.enterprisedb.com