On Tue, Feb 12, 2019 at 10:42 AM Tom Lane <tgl@xxxxxxxxxxxxx> wrote:
Thomas Kellerer <spam_eater@xxxxxxx> 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
