Re: vacuum, performance, and MVCC

Ühel kenal päeval, R, 2006-06-23 kell 17:27, kirjutas Bruce Momjian:
> Jonah H. Harris wrote:
> > On 6/23/06, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> > > What I see in this discussion is a huge amount of "the grass must be
> > > greener on the other side" syndrome, and hardly any recognition that
> > > every technique has its downsides and complications.
> >
> > I'm being totally objective. I don't think we should abandon
> > PostgreSQL's overall design at all, because we do perform INSERTs and
> > DELETEs much better than most systems. However, I've looked at many
> > systems and how they implement UPDATE so that it is a scalable
> > operation. Sure, there are costs and benefits to each implementation,
> > but I think we have some pretty brilliant people in this community and
> > can come up with an elegant design for scalable UPDATEs.
>
> I think the UPDATE case is similar to the bitmap index scan or perhaps
> bitmap indexes on disk --- there are cases we know can not be handled
> well by our existing code, so we have added (or might add) these
> features to try to address those difficult cases.

Not really. Bitmap index scan and bitmap index are both new additions
working well with existing framework.

While the problem of slowdown on frequent updates is real, the suggested
fix is just plain wrong, as it is based on someones faulty assumption on
how index lookup works, and very much simplified view of how different
parts of the system work to implement MVCC.

The original fix he "suggests" was to that imagined behaviour and thus
ignored all the real problems of such change.

All the next suggestions were variations of the first ones, and failed
to address or even research any problems brought up.

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Hannu Krosing
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