Re: Synchronized scans

On Mon, 2007-06-04 at 16:42 -0400, Tom Lane wrote:
> Heikki Linnakangas <heikki@enterprisedb.com> writes:
> > I don't think anyone can reasonably expect to get the same ordering when
> > the same query issued twice in general, but within the same transaction
> > it wouldn't be that unreasonable. If we care about that, we could keep
> > track of starting locations per transaction, only do the synchronization
> > on the first scan in a transaction, and start subsequent scans from the
> > same page as the first one.
>
> I think the real problem here is that the first scan is leaving state
> behind that changes the behavior of the next scan.  Which can have no
> positive benefit, since obviously the first scan is not still
> proceeding; the best you can hope for is that it's a no-op and the worst
> case is that it actively pessimizes things.  Why doesn't the patch
> remove the shmem entry at scan termination?
>

Sounds like a reasonable idea to me. We could add the PID to the data
structure so that it would only remove the hint if it's the one that set
the hint. I think we'd just need to call a function to do that from
heap_endscan(), correct?

However, we couldn't 100% guarantee that the state would be cleared. A
backend could be killed in the middle of a scan.

The case you're worried about seems very narrow to me, and I think
"actively pessimizes" is too strong. Unless I misunderstand, "the best
you can hope for" no-op happens in all cases except a most bizarre one:
that in which you're executing repeated identical LIMIT queries with no
ORDER BY; and the tuples returned occupy more than 128K (16 pages is the
reporting period) but fewer would be effective to cache; and the table
in question is larger than the large table threshold. I'm just trying to
add some perspective about what we're fixing, here.

But it's fair to say that a scan should clear any state when it's done.

Regards,
    Jeff Davis