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ROLLUP (a, b, c) is equivalent to GROUPING SETS ( (a, b, c), (a, b), (a), ( )).
The general syntax of ROLLUP is ROLLUP( )

hive.new.job.grouping.set.cardinality

Whether a new map-reduce job should be launched for grouping sets/rollups/cubes.
For a query like: select a, b, c, count(1) from T group by a, b, c with rollup;
4 rows are created per row: (a, b, c), (a, b, null), (a, null, null), (null, null, null)
This can lead to explosion across map-reduce boundary if the cardinality of T is very high
and map-side aggregation does not do a very good job.

This parameter decides if hive should add an additional map-reduce job. If the grouping set
cardinality (4 in the example above), is more than this value, a new MR job is added under the
assumption that the orginal group by will reduce the data size.