On Tue, Mar 21, 2017 at 10:47 PM, Bruce Momjian <br...@momjian.us> wrote:

> On Tue, Mar 21, 2017 at 01:04:14PM -0400, Robert Haas wrote:
> > > I know we have talked about it, but not recently, and if everyone else
> > > is fine with it, I am too, but I have to ask these questions.
> >
> > I think that's a good question.  I previously expressed similar
> > concerns.  On the one hand, it's hard to ignore the fact that, in the
> > cases where this wins, it already buys us a lot of performance
> > improvement.  On the other hand, as you say (and as I said), it eats
> > up a lot of bits, and that limits what we can do in the future.  On
> > the one hand, there is a saying that a bird in the hand is worth two
> > in the bush.  On the other hand, there is also a saying that one
> > should not paint oneself into the corner.
> >
> > I'm not sure we've had any really substantive discussion of these
> > issues.  Pavan's response to my previous comments was basically "well,
> > I think it's worth it", which is entirely reasonable, because he
> > presumably wouldn't have written the patch that way if he thought it
> > sucked.  But it might not be the only opinion.
> Early in the discussion we talked about allowing multiple changes per
> WARM chain if they all changed the same index and were in the same
> direction so there were no duplicates, but it was complicated.  There
> was also discussion about checking the index during INSERT/UPDATE to see
> if there was a duplicate.  However, those ideas never led to further
> discussion.

Well, once I started thinking about how to do vacuum etc, I realised that
any mechanism which allows unlimited (even handful) updates per chain is
going to be very complex and error prone. But if someone has ideas to do
that, I am open. I must say though, it will make an already complex problem
even more complex.

> I know the current patch yields good results, but only on a narrow test
> case,

Hmm. I am kinda surprised you say that because I never thought it was a
narrow test case that we are targeting here. But may be I'm wrong.


 Pavan Deolasee                   http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services

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