Re: [HACKERS] [PROPOSAL] Use SnapshotAny in get_actual_variable_range

Robert Haas <robertmhaas@gmail.com> writes:
> On Thu, Apr 27, 2017 at 5:22 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> How so?  Shouldn't the indexscan go back and mark such tuples dead in
>> the index, such that they'd be visited this way only once?  If that's
>> not happening, maybe we should try to fix it.

> Hmm.  Actually, I think the scenario I saw was where there was a large
> number of tuples at the end of the index that weren't dead yet due to
> an old snapshot held open.  That index was being scanned by lots of
> short-running queries.  Those queries executed just fine, but they
> took a long to plan because they had to step over all of the dead
> tuples in the index one by one.

But that was the scenario that we intended to fix by changing to
SnapshotDirty, no?  Or I guess not quite, because
dead-but-still-visible-to-somebody tuples are rejected by SnapshotDirty.

Maybe we need another type of snapshot that would accept any
non-vacuumable tuple.  I really don't want SnapshotAny semantics here,
but a tuple that was live more recently than the xmin horizon seems
like it's acceptable enough.  HeapTupleSatisfiesVacuum already
implements the right behavior, but we don't have a Snapshot-style
interface for it.
        regards, tom lane