Redis hyperlog keys cardinality is not increasing

Question

When using a hyperlog key in redis master slave 4.0.9 we have a pfcount of 52161862.

Now when we add a unique item through pfadd it is returning zero and of count is still 52161862.

Any idea why we are not able to add more unique items to this key?

Answer 1

Hyperloglog is NOT an accurate counting, instead, the result of PFCOUNT is approximated with a standard error of 0.81%. So when you add a new unique item, the result might be unchanged.