Re: Bloom index cost model seems to be wrong

On Tue, Feb 12, 2019 at 10:42 AM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> Thomas Kellerer <spam_eater(at)gmx(dot)net> writes:
> > The bloom index is only used if either Seq Scan is disabled or if the
> random_page_cost is set to 1 (anything about 1 triggers a Seq Scan on my
> Windows laptop).
>
> Hm. blcostestimate is using the default cost calculation, except for
>
> /* We have to visit all index tuples anyway */
> costs.numIndexTuples = index->tuples;
>
> which essentially tells genericcostestimate to assume that every index
> tuple will be visited. This obviously is going to increase the cost
> estimate; maybe there's something wrong with that?
>

I assumed (without investigating yet) that genericcostestimate is applying
a cpu_operator_cost (or a few of them) on each index tuple, while the
premise of a bloom index is that you do very fast bit-fiddling, not more
expense SQL operators, for each tuple and then do the recheck only on what
survives to the table tuple part.

Cheers,

Jeff