Re: INSERT...ON DUPLICATE KEY LOCK FOR UPDATE

On 2013-10-15 10:53:35 -0700, Peter Geoghegan wrote:
> On Tue, Oct 15, 2013 at 10:29 AM, Andres Freund <andres@2ndquadrant.com> wrote:
> > I think anything that only works by breaking visibility rules that way
> > is a nonstarter. Doing that from the C level is one thing, exposing it
> > this way seems a bad idea.
> 
> What visibility rule is that?

The early return you added to HTSMVCC.

At the very least it opens you to lots of halloween problem like
scenarios.

> Upsert *has* to do effectively the same thing as what I've proposed -
> there is no getting away from it. So maybe the visibility rulebook
> (which as far as I can tell is "the way things work today") needs to
> be updated. If we did, say, INSERT...ON DUPLICATE KEY UPDATE, we'd
> have to update a row with potentially no visible-to-snapshot version
> *at all*, and make a new version of that visible. That's just what it
> takes. What's the difference between that and just locking? If the
> only difference is that it isn't necessary to modify tqual.c because
> you're passing a tid directly, that isn't a user-visible difference -
> the "rule" has been broken just the same.  Arguably, it's even more of
> a hack, since it's a special, out-of-band visibility exception.

No, doing it in special case code is fundamentally different since those
locations deal only with one row at a time. There's no scans that can
pass over that row.
That's why I think exposing the "on conflict lock" logic to anything but
C isn't going to fly btw.

Greetings,

Andres Freund

-- Andres Freund                       http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training &
Services