Re: Slow query problem

On Fri, 9 Jan 2004, Richard Huxton wrote:

> On Friday 09 January 2004 08:57, Dennis Bjrklund wrote:
> > On Fri, 9 Jan 2004, Richard Huxton wrote:
> > > > > select invheadref, invprodref, sum(units)
> > > > > from invtran
> > > > > group by invheadref, invprodref
> > > >
> > > > For the above query, shouldn't you have one index for both columns
> > > > (invheadref, invprodref). Then it should not need to sort at all to do
> > > > the grouping and it should all be fast.
> > >
> > > Not sure if that would make a difference here, since the whole table is
> > > being read.
> >
> > The goal was to avoid the sorting which should not be needed with that
> > index (I hope). So I still think that it would help in this case.
>
> Sorry - not being clear. I can see how it _might_ help, but will the planner
> take into account the fact that even though:
>   index-cost > seqscan-cost
> that
>   (index-cost + no-sorting) < (seqscan-cost + sort-cost)
> assuming of course, that the costs turn out that way.

AFAICS, yes it does take that effect into account (as best
it can with the estimates).