Re: Bloom index cost model seems to be wrong
| От | Jeff Janes |
|---|---|
| Тема | Re: Bloom index cost model seems to be wrong |
| Дата | |
| Msg-id | CAMkU=1x9ML8PPmPKM0x7OVv6oUFn4b8s50suj2jcCOT78TLkzQ@mail.gmail.com обсуждение исходный текст |
| Ответ на | Re: Bloom index cost model seems to be wrong (Tom Lane <tgl@sss.pgh.pa.us>) |
| Ответы |
Re: Bloom index cost model seems to be wrong
|
| Список | pgsql-performance |
On Tue, Feb 12, 2019 at 10:42 AM Tom Lane <tgl@sss.pgh.pa.us> wrote:
Thomas Kellerer <spam_eater@gmx.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
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