Re: Vacuum threshold and non-serializable read-only transaction

ITAGAKI Takahiro <itagaki(dot)takahiro(at)oss(dot)ntt(dot)co(dot)jp> writes:
> I think we can remove recently dead tuples even if non-serializable read-only
> transactions are still alive, because those transactions will not see older
> versions of tuples.

Surely this'd require having those transactions display exactly what
their current oldest-xmin is. We've talked about that before, and it
seems a good idea, but it requires a bit more infrastructure than is
there now --- we'd need some snapshot-management code that could keep
track of all live snapshots within each backend.

> Is it proper behavior? I worry about too conservative estimation
> in incrementing ShmemVariableCache->latestCompletedXid.

Too conservative is much better than too liberal, in this case
(and I'm as bleeding-heart liberal as they come ;-))

regards, tom lane