Explanation/Reference:
Limit the number of nodes that process a query, typically during debugging. Only accepts the values 0 (meaning all nodes) or 1 (meaning all work is done on the coordinator node). If you are diagnosing a problem that you suspect is due to a timing issue due to distributed query processing, you can set NUM_NODES=1 to verify if the problem still occurs when all the work is done on a single node.
Incorrect Answers:
A: the query option DISABLE_UNSAFE_SPILLS=true limits the spill-to-disk feature to prevent runaway disk usage from queries that are known in advance to be suboptimal.
References: http://www.cloudera.com/documentation/cdh/5-1-x/Impala/Installing-and-Using-Impala/ ciiu_num_nodes.html