Re: tweaking NTUP_PER_BUCKET

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.

regards
Tomas