Re: [PATCH] Full support for index LP_DEAD hint bits on standby

On Mon, Mar 28, 2022 at 1:23 PM Peter Geoghegan <pg(at)bowt(dot)ie> wrote:
> I doubt that the patch's use of pg_memory_barrier() in places like
> _bt_killitems() is correct.

I also doubt that posting list splits are handled correctly.

If there is an LP_DEAD bit set on a posting list on the primary, and
we need to do a posting list split against the posting tuple, we need
to be careful -- we cannot allow our new TID to look like it's LP_DEAD
immediately, before our transaction even commits/aborts. We cannot
swap out our new TID with an old LP_DEAD TID, because we'll think that
our new TID is LP_DEAD when we shouldn't.

This is currently handled by having the inserted do an early round of
simple/LP_DEAD index tuple deletion, using the "simpleonly" argument
from _bt_delete_or_dedup_one_page(). Obviously the primary cannot be
expected to know that one of its standbys has independently set a
posting list's LP_DEAD bit, though. At the very least you need to
teach the posting list split path in btree_xlog_insert() about all
this -- it's not necessarily sufficient to clear LP_DEAD bits in the
index AM's fpi_mask() routine.

Overall, I think that this patch has serious design flaws, and that
this issue is really just a symptom of a bigger problem.

--
Peter Geoghegan