I'm not too familiar with parallel planning, but I tried to implement both in attached patch. I didn't put much effort into the parallel_threshold GUC documentation, because I didn't really see a good way to explain it. I'd e happy to improve it if needed. Also, to make this parameter easier to tune for users, perhaps we could divide the default value by 3 and use it as is in the first iteration in create_parallel_path() ?
Also, global max_parallel_degree still needs to be at least 1 for the per table value to be considered.
All applies and works from my end.
Is the max_parallel_degree per table of much use here? It allows the max number of workers per table to be set - but it's still bound by the same formula (now from the GUC). So in reality it's only really useful for limiting the number of workers, not raising it.
Would there be a common use case for limiting parallelism on a subset of tables in a database you've explicitly set to have a higher amount of parallel operations via the GUC? I struggle to think of one?
I think in practicality the reverse would be more common, you'd want to set certain tables to a starting point of a certain number of workers (and ramp up to more if the formula allowed it). You could set this to 0 for never use parallel agg on this table.
Another option is to allow access to the the threshold multiplier (currently hard coded to 3) per table - but this might become pretty hard to explain succinctly in the documentation.
Cheers,
James
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