Counting cardinality estimations may become negative

[KMax]

The card(c), card(p) and card(p, o) may be below zero, which is wrong. Maybe fixed by using the conditional UPDATE which would check if the counter is not less than the decrement.

[KMax]

INSERT ... IF NOT EXISTS employs the lightweight transactions mechanism which isn't supported by ScyllaDB yet (see scylladb/scylladb#1359). And lightweight transactions introduce a significant overhead which may be undesirable.