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This is a question that the MapR Drill team is often asked.
Parallelism, or the number of parallel processing threads working on a query is controlled by the planner.width.max_per_node option, which can be altered at the session level. The default will match 70% of the number of CPU cores. Decreasing this setting decreases parallelism. E.g.,:
alter session set planner.width.max_per_node=10;
For people who needs more details on controlling query parallelization, here is one article explaining about below 3:
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