Re: BUG #17255: Server crashes in index_delete_sort_cmp() due to race condition with vacuum

On Thu, Mar 10, 2022 at 8:08 PM Andres Freund <andres@anarazel.de> wrote:
> But that's probably best done separately - but perhaps worth to "weaken" the
> comment a bit?

I'm confused again.

I don't get why it's only going to be worth delaying establishing
vistest if we can also delay establishing OldestXmin in about the same
way. AFAICT the only compelling reason to have two separate cutoffs
from the point of view of vacuumlazy.c is that it enables periodically
refreshing vistest within lazy_scan_prune, to allow it to prune dead
tuples more eagerly (fewer recently dead tuples, more dead removed
tuples). That I can see, that much I get.

Periodically refreshing OldestXmin for freezing won't work, though. At
the very least it seems much less compelling than the vistest idea.
Yes, it probably is true that we could in principle further split
OldestXmin along "xmin vs xid horizon" lines (I should say "split it
again" -- you already split OldestXmin into "OldestXmin for freezing,
vistest for pruning" as part of your snapshot scalability work). But
would it really make any sense? If so, I can't see why. At least not
right now.

--
Peter Geoghegan