Re: Re: why postgresql define NTUP_PER_BUCKET as 10, not other numbers smaller
| От | Jeff Janes |
|---|---|
| Тема | Re: Re: why postgresql define NTUP_PER_BUCKET as 10, not other numbers smaller |
| Дата | |
| Msg-id | CAMkU=1y3KGb3_SLN8H5X46B3Q-Xupnr=ogAF_hcQbYGW0Un1og@mail.gmail.com обсуждение исходный текст |
| Ответ на | Re: Re: why postgresql define NTUP_PER_BUCKET as 10, not other numbers smaller (Robert Haas <robertmhaas@gmail.com>) |
| Ответы |
Re: Re: why postgresql define NTUP_PER_BUCKET as 10, not
other numbers smaller
|
| Список | pgsql-hackers |
On Tue, Jun 10, 2014 at 5:17 AM, Robert Haas <robertmhaas@gmail.com> wrote:
The problem case is when you have 1 batch and the increased memory
consumption causes you to switch to 2 batches. That's expensive. It
seems clear based on previous testing that *on the average*
NTUP_PER_BUCKET = 1 will be better, but in the case where it causes an
increase in the number of batches it will be much worse - particularly
because the only way we ever increase the number of batches is to
double it, which is almost always going to be a huge loss.
Is there a reason we don't do hybrid hashing, where if 80% fits in memory than we write out only the 20% that doesn't? And then when probing the table with the other input, the 80% that land in in-memory buckets get handled immediately, and only the 20 that land in the on-disk buckets get written for the next step?
Obviously no one implemented it yet, but is there a fundamental reason for that or just a round tuit problem?
Cheers,
Jeff
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