Re: Vacuum ERRORs out considering freezing dead tuples from before OldestXmin

On Mon, Jun 24, 2024 at 3:36 PM Melanie Plageman
<melanieplageman(at)gmail(dot)com> wrote:
> One thing I don't understand is why it is okay to freeze the xmax of a
> dead tuple just because it is from an aborted update.

We don't do that with XID-based xmaxs. Though perhaps we should, since
we'll already prune-away the successor tuple, and so might as well go
one tiny step further and clear the xmax for the original tuple via
freezing/setting it InvalidTransactionId. Instead we just leave the
original tuple largely undisturbed, with its original xmax.

We do something like that with Multi-based xmax fields, though not
with the specific goal of cleaning up after aborts in mind (we can
also remove lockers that are no longer running, regardless of where
they are relative to OldestXmin, stuff like that). The actual goal
with that is to enforce MultiXactCutoff, independently of whether or
not their member XIDs are < FreezeLimit yet.

> The only case in which we freeze dead tuples
> with a non-multi xmax is if the xmax is from before OldestXmin and is
> also not committed (so from an aborted update).

Perhaps I misunderstand, but: we simply don't freeze DEAD (not
RECENTLY_DEAD) tuples in the first place, because we don't have to
(pruning removes them instead). It doesn't matter if they're DEAD due
to being from aborted transactions or DEAD due to being
deleted/updated by a transaction that committed (committed and <
OldestXmin).

The freezing related code paths in heapam.c don't particularly care
whether a tuple xmax is RECENTLY_DEAD or LIVE to HTSV + OldestXmin.
Just as long as it's not fully DEAD (then it should have been pruned).

--
Peter Geoghegan