PFMERGE destkey sourcekey [sourcekey ...]
Merge multiple HyperLogLog values into an unique value that will approximate the cardinality of the union of the observed Sets of the source HyperLogLog structures.
The computed merged HyperLogLog is set to the destination variable, which is created if does not exist (defaulting to an empty HyperLogLog).
Simple string reply: The command just returns
(integer) 1redis> PFADD hll2 a b c foo
(integer) 1redis> PFMERGE hll3 hll1 hll2
"OK"redis> PFCOUNT hll3
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