Re: tweaking NTUP_PER_BUCKET

On 19.7.2014 20:24, Tomas Vondra wrote:
> On 13.7.2014 21:32, Tomas Vondra wrote:
>> The current patch only implemnents this for tuples in the main
>> hash table, not for skew buckets. I plan to do that, but it will
>> require separate chunks for each skew bucket (so we can remove it
>> without messing with all of them). The chunks for skew buckets
>> should be smaller (I'm thinking about ~4kB), because there'll be
>> more of them, but OTOH those buckets are for frequent values so the
>> chunks should not remain empty.
>
> I've looked into extending the dense allocation to the skew buckets,
> and I think we shouldn't do that. I got about 50% of the changes and
> then just threw it out because it turned out quite pointless.
>
> The amount of memory for skew buckets is limited to 2% of work mem,
> so even with 100% overhead it'll use ~4% of work mem. So there's
> pretty much nothing to gain here. So the additional complexity
> introduced by the dense allocation seems pretty pointless.
>
> I'm not entirely happy with the code allocating some memory densely
> and some using traditional palloc, but it certainly seems cleaner
> than the code I had.
>
> So I think the patch is mostly OK as is.

Attached is v4 of the patch, mostly with minor improvements and cleanups
(removed MemoryContextStats, code related to skew buckets).

regards
Tomas