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

On 2013-09-22 12:54:57 -0700, Peter Geoghegan wrote:
> On Sun, Sep 22, 2013 at 2:10 AM, Andres Freund <andres@2ndquadrant.com> wrote:
> > I can't follow here. Why does e.g. the promise tuple approach bloat more
> > than the subxact example?
> > The protocol is roughly:
> > 1) Insert index pointer containing an xid to be waiting upon instead of
> >    the target tid into all indexes
> > 2) Insert heap tuple, we can be sure there's no conflict now
> > 3) Go through the indexes and repoint the item to point to the tid of the
> >    heaptuple instead of the xid.
> >
> > There's zero heap or index bloat in the uncontended case. In the
> > contended case it's just the promise tuples from 1) that are inserted
> > before the conflict is detected. Those can be marked as dead when the
> > conflict happened.
> 
> It depends on your definition of the contended case. You're assuming
> that insertion is the most probable outcome, when in fact much of the
> time updating is just as likely or even more likely. Many promise
> tuples may be inserted before actually seeing a conflict and deciding
> to update/lock for update. 

I still fail to see how that's relevant. For every index there's two
things that can happen:
a) there's a conflicting tuple. In that case we can fail at that
point/convert to an update. No Bloat.
b) there's no conflicting tuple. In that case we will insert a promise
tuple. If there's no conflict in further indexes (i.e. we INSERT), the
promise will converted to a plain tuple. If there *is* a further
conflict, you *still* need the new index tuple because by definition
(the index changed) it cannot be an HOT update. So you convert it as
well. No Bloat.

> I think that bloat that is generally cleaned up synchronously is still
> bloat

I don't think it's particularly relevant because the above will just
cause bloat in case of rollbacks and such which is nothin new, but:
I fail to fee the point of such a position.

> I think reviewer time would for now be much better spent discussing
> the patch at a higher level (along the lines of my recent mail to
> Stephen and Robert).

Yes, I plan to reply to those, I just didn't have time to do so this
weekend. There's other stuff than PG every now and then ;)

Greetings,

Andres Freund

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