When selecting and inserting/updating columns, one can get/set the TIMESTAMP / WRITETIME and TTL. However, this is not enough information to recreate the column's expiration time (clock sync and network latency between the client and server get in the way). This makes updating columns with a set TTL fragile: there is no way to make the new value of a column expire at the same time as the old value.
Ideally, you'd be able to say something like:
SELECT x, y EXPIRATIONTIME( y ) FROM my_cf
UPDATE my_cf USING EXPIRATIONTIME sameAsFromSelect SET y = newy WHERE x = oldx.
The use case I'm facing is that I write an entire row with a given TTL, and might later want to update a few of its columns. Currently, that makes the updated columns live longer than the non-updated columns. Of course, you can come up with a good approximation for the appropriate TTL in the update to make the updated columns expire at around the same time, but not at exactly the same time. Since Cassandra stores an expiration time internally, making the expiration exactly simultaneous should be possible, but CQL3 does not expose this ability.