About expiring columns
Data in a column can have an optional expiration date called TTL (time to live). The client request specifies a TTL value, defined in seconds, for the data. TTL data is marked with a tombstone after the requested amount of time has expired. After data is marked with a tombstone, the data is automatically removed during the normal compaction defined by the gc_grace_seconds and repair processes.
Use CQL to set the TTL for data.
If you want to change the TTL of expiring data, you have to re-insert the data with a new TTL. In Cassandra, the insertion of data is actually an insertion or update operation, depending on whether or not a previous version of the data exists.
TTL data has a precision of one second, as calculated on the server. Therefore, a very small TTL probably does not make much sense. Moreover, the clocks on the servers should be synchronized; otherwise reduced precision could be observed because the expiration time is computed on the primary host that receives the initial insertion but is then interpreted by other hosts on the cluster.
Expiring data has an additional overhead of 8 bytes in memory and on disk (to record the TTL and expiration time) compared to standard data.