On 03/25/2017 02:01 PM, David Rowley wrote: >
I wondered if there's anything we can do here to better test cases when no workers are able to try to ensure the parallel nodes work correctly, but the more I think about it, the more I don't see wrong with just using SET max_parallel_workers = 0;
It's demonstrably a valid way to disable parallel queries (i.e. there's nothing wrong with it), because the docs say this:
Setting this value to 0 disables parallel query execution. >
My vote would be to leave the GUC behaviour as is and add some tests to select_parallel.sql which exploit setting max_parallel_workers to 0 for running some tests. If that's not going to fly, then unless we add something else to allow us to reliably not get any workers, then we're closing to close the door on being able to write automatic tests to capture this sort of bug. ... thinks for a bit.... perhaps some magic value like -1 could be used for this... unsure of how that would be documented though...
I agree it'd be very useful to have a more where we generate parallel plans but then prohibit starting any workers. That would detect this and similar issues, I think.
I'm not sure we need to invent a new magic value, though. Can we simply look at force_parallel_mode, and if it's 'regress' then tread 0 differently?
regards -- Tomas Vondra http://www.2ndQuadrant.com PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services -- Sent via pgsql-hackers mailing list (email@example.com) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers