ALTER TABLE table_name ADD PARTITION partition_column | hidden_partition_spec [AS alias]

hidden_partition_spec:
    Supported transformations are:
        years(ts): partition by year
        months(ts): partition by month
        days(ts) or date(ts): equivalent to dateint partitioning
        hours(ts) or date_hour(ts): equivalent to dateint and hour partitioning
        bucket(N, col): partition by hashed value mod N buckets
        truncate(L, col): partition by value truncated to L
            Strings are truncated to the given length
            Integers and longs truncate to bins: truncate(10, i) produces partitions 0, 10, 20, 30, …

ALTER TABLE prod.db.sample ADD PARTITION FIELD bucket(16, id)
ALTER TABLE prod.db.sample ADD PARTITION FIELD truncate(data, 4)
ALTER TABLE prod.db.sample ADD PARTITION FIELD years(ts)
-- use optional AS keyword to specify a custom name for the partition field 
ALTER TABLE prod.db.sample ADD PARTITION FIELD bucket(16, id) AS shard