Re: PATCH: Extending the HyperLogLog API a bit

On Tue, Jan 19, 2016 at 9:37 AM, Alvaro Herrera
<alvherre@2ndquadrant.com> wrote:
> Our transcript seems to predate that bugfix commit, so I assume we need
> to apply this to our copy too.  Sadly, Hideaki-san commit message isn't
> very descriptive.

Fortunately, the function mergeHyperLogLog() in our hyperloglog.c
currently has no callers.

> I don't really know how HyperLogLog works, so maybe we can't or
> shouldn't apply the patch because of how the hash stuff is used.

I think that Hideaki's confusion comes from whether or not this HLL
state is a sparse or dense/full representation. The distinction is
explained in the README for postgresql-hll:

https://github.com/aggregateknowledge/postgresql-hll

postgresql-hll has no support for merging HLLs that are sparse:

https://github.com/aggregateknowledge/postgresql-hll/blob/master/hll.c#L1888

Can't we just tear mergeHyperLogLog() out?

-- 
Peter Geoghegan