Re: ALTER TABLE lock strength reduction patch is unsafe

Robert Haas <robertmhaas@gmail.com> writes:
> On Tue, Jan 3, 2012 at 1:42 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> BTW, I wonder if this couldn't be ameliorated by establishing some
>> ground rules about how up-to-date a snapshot really needs to be.
>> Arguably, it should be okay for successive SnapshotNow scans to use the
>> same snapshot as long as we have not acquired a new lock in between.
>> If not, reusing an old snap doesn't introduce any race condition that
>> wasn't there already.

> Is that likely to help much?  I think our usual pattern is to lock the
> catalog, scan it, and then release the lock, so there will normally be
> an AcceptInvalidationMessages() just before the scan.  Or at least, I
> think there will.

Um, good point.  Those locks aren't meant to avoid race conditions,
but the mechanism doesn't know that.

> Another thought is that it should always be safe to reuse an old
> snapshot if no transactions have committed or aborted since it was
> taken

Yeah, that might work better.  And it'd be a win for all MVCC snaps,
not just the ones coming from promoted SnapshotNow ...
        regards, tom lane