Using force_parallel_mode Correctly

I admit it: I invented force_parallel_mode.  I believed then, and still believe now, that it is valuable for testing purposes.  Certainly, testing using force_parallel_mode=on or force_parallel_mode=regress has uncovered many bugs in PostgreSQL's parallel query support that would otherwise have been very difficult to find.  At the same time, it's pretty clear that this setting has caused enormous confusion, even among PostgreSQL experts.  In fact, in my experience, almost everyone who sets force_parallel_mode is doing so for the wrong reasons.

PostgreSQL Regression Test Coverage

Yesterday evening, I ran the PostgreSQL regression tests (make check-world) on master and on each supported back-branch three times on hydra, a community test machine provided by IBM.  Here are the median results:

9.1 - 3m49.942s
9.2 - 5m17.278s
9.3 - 6m36.609s
9.4 - 9m48.211s
9.5 - 8m58.544s
master, or 9.6 - 13m16.762s