Re: Fix gin index cost estimation

Hi, Ronan!

On Wed, Oct 12, 2022 at 10:15 AM Ronan Dunklau <ronan(dot)dunklau(at)aiven(dot)io>
wrote:
> > > You're right, I was too eager to try to raise the CPU cost
proportionnally
> to
> > > the number of array scans (scalararrayop). I'd really like to
understand
> where
> > > this equation comes from though...
> >
> > So, what's the latest update here?
>
> Thanks Michael for reviving this thread.
>
> Before proceeding any further with this, I'd like to understand where we
> stand. Tom argued my way of charging cost per entry pages visited boils
down
> to charging per tuple, which I expressed disagreement with.
>
> If we can come to a consensus whether that's a bogus way of thinking
about it
> I'll proceed with what we agree on.

I briefly read the thread. I think this line is copy-paste from other index
access methods and trying to estimate the whole index scan CPU cost by
bypassing all the details.

*indexTotalCost += (numTuples * *indexSelectivity) * (cpu_index_tuple_cost
+ qual_op_cost);

I think Tom's point was that it's wrong to add a separate entry-tree CPU
cost estimation to another estimation, which tries (very inadequately) to
estimate the whole scan cost. Instead, I propose writing better estimations
for both entry-tree CPU cost and data-trees CPU cost and replacing existing
CPU estimation altogether.

------
Regards,
Alexander Korotkov