I noticed that the joins in TestTpchDistributed unit tests are not getting parallelized. The aggregations and sort are getting parallelized. I have filed DRILL-2900. We do have test coverage for parallel joins elsewhere, especially with TPCH SF100 tests; so this is an FYI in case you are relying on TestTpchDistributed for such coverage.
This also raises the importance of plan tracking for more tests. We have been doing plan tracking for partition pruning tests and it has worked out quite well. Aman
