Re: snapshot too old issues, first around wraparound and then more.

Hi,

On 2020-04-01 16:59:51 -0700, Andres Freund wrote:
> The primary issue here is that there is no TestForOldSnapshot() in
> heap_hot_search_buffer(). Therefore index fetches will never even try to
> detect that tuples it needs actually have already been pruned away.

FWIW, with autovacuum=off the query does not get killed until a manual
vacuum, nor if fewer rows are deleted and the table has previously been
vacuumed.

The vacuum in the second session isn't required. There just needs to be
something consuming an xid, so that oldSnapshotControl->latest_xmin is
increased. A single SELECT txid_current(); or such in a separate session
is sufficient.

Greetings,

Andres Freund