On Tue, Jan 19, 2016 at 9:37 AM, Alvaro Herrera
> 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:
postgresql-hll has no support for merging HLLs that are sparse:
Can't we just tear mergeHyperLogLog() out?
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