Hi @divingdiana
The alter table to set the TTL should be an online operation. Furthermore, it is a meta-data only update that is done to all the shards (tablets), and doesn’t require reading or re-writing all the data. So the operation should be pretty quick and is not dependent on the data size.
To your second question: The TTL set via the alter table applies retroactively to older/existing data in the table also, not just to newer data.
regards,
Kannan