As DuyHai said, different TTLs could theoretically be set for different
cells of the same row. And one TTLed cell could be shadowing another cell
that has no TTL (say you forgot to set a TTL and set one afterwards by
performing an update), or vice versa.
One cell could also be missing from a node without Cassandra knowing. So
turning an incomplete row that only has expired cells into a tombstone row
could lead to wrong results being returned at read time : the tombstone row
could potentially shadow a valid live cell from another replica.

Cassandra needs to retain each TTLed cell and send it to replicas during
reads to cover all possible cases.
On Fri, Jan 12, 2018 at 5:28 PM Python_Max <[EMAIL PROTECTED]> wrote:
Alexander Dejanovski

Apache Cassandra Consulting