Carter Shanklin introduces the MERGE operator in Hive:
USE CASE 2: UPDATE HIVE PARTITIONS.
A common strategy in Hive is to partition data by date. This simplifies data loads and improves performance. Regardless of your partitioning strategy you will occasionally have data in the wrong partition. For example, suppose customer data is supplied by a 3rd-party and includes a customer signup date. If the provider had a software bug and needed to change customer signup dates, suddenly records are in the wrong partition and need to be cleaned up.
It has been interesting to see Hive morph over the past few years from a batch warehousing system to something approaching a relational warehouse. This is one additional step in that direction.