On Sun, Dec 8, 2019 at 1:24 PM Jeff Janes <jeff.ja...@gmail.com> wrote: > Is there a way to force a meaningful parallel seq scan, or at least the > planning of one, when the planner wants a non-parallel one? > > Usually I can do things like with with enable_* setting, but if I `set > enable_seqscan to off`, it penalizes the parallel seq scan 8 times harder > than it penalizes the non-parallel one, so the plan does not switch. > > If I set `force_parallel_mode TO on` then I do get a parallel plan, but it is > a degenerate one which tells me nothing I want to know. > > If I `set parallel_tuple_cost = 0` (or in some cases to a negative number), I > can force it switch, but that destroys the purpose, which is to see what the > "would have been" plan estimates are for the parallel seq scan under the > default setting of the cost parameters. > > I can creep parallel_tuple_cost downward until it switches, and then try to > extrapolate back up, but this tedious and not very reliable.
I don't think there's a way to force this, but setting both parallel_setup_cost and parallel_tuple_cost to 0 seems to often be enough. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company
