maytasm opened a new issue, #735:
URL: https://github.com/apache/datasketches-java/issues/735
There is no efficient way to merge two HLL Unions together.
HLL Union has the method update but that only takes `HllSketch`
To get a `HllSketch` from a HLL Union, we need to call HLL Union `getResult`
which then calls `gadget.copyAs(tgtHllType);`
However, this does a deep copy, slowing down the merge.
Basically,
when this and other are both HLL Union:
→ other.union.getResult() // line 85: triggers gadget.copyAs() —
FULL COPY of 65K registers
→ union.update(copiedSketch) // line 111: iterates 65K registers
again to merge
Two passes over 65K registers + one allocation, when only one pass is needed.
Am I missing something? Thanks!
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