HyperLogLog uses additions, it keeps sums. Thus, you can subtract one HLL sums from other. This is useful if stream supports deletion. Streams with deletions ca

HyperLogLog uses additions, it keeps sums. Thus, you can subtract one HLL sums from other. This is useful if stream supports deletion. Streams with deletions can be found in log-structured merge trees, for one example, so one can estimate count of distinct elements in all of the LSM tree hierarchy. The algorithm in the paper does not allow for deletions. Also, if one counts statistics of the stream of large elements (say, SHA-512 hashes, 64 bytes per hash), this algorithm requires some storage for elements from this stream, so memory requirement is O(table size * element size).