I decided to try out parallel query, as implemented in PostgreSQL 9.6devel, on the TPC-H queries. To do this, I followed the directions at https://github.com/tvondra/pg_tpch - thanks to Tomas Vondra for those instructions. I did the test on an IBM POWER7 server provided to the PostgreSQL community by IBM. I scaled the database to use 10GB of input data; the resulting database size was 22GB, of which 8GB was indexes. I tried out each query just once without really tuning the database at all, except for increasing shared_buffers to 8GB. Then I tested them again after enabling parallel query by configuring max_parallel_degree = 4.