Re: how does PostgreSQL determine how many parallel processes to start

On Fri, 2021-02-19 at 10:38 +0100, Luca Ferrari wrote:
> I know that parallel processes can be limited by
> max_parallel_workers_per_gather and max_parallel_workers, as well as
> the condition to consider a parallel plan is min_table_scan_size (and
> index). But I would like to understand, once a table has been
> considered for a parallel plan, and there is room for other workers,
> how will PostgreSQL decide to start another process?

During planning, it will generate parallel and non-parallel plans
and take the one it estimates to be cheapest.

At execution time, PostgreSQL will use as many of the planned workers
as are currently available (max_parallel_workers).

Yours,
Laurenz Albe
-- 
Cybertec | https://www.cybertec-postgresql.com