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

On Fri, 2021-02-19 at 11:21 +0100, Luca Ferrari wrote:
> > At execution time, PostgreSQL will use as many of the planned workers
> > as are currently available (max_parallel_workers).
> 
> Thanks, but just to make it clear, assuming I execute almost
> simultanously two identical queries that can be therefore be
> parallelized, does that mean that the first one will be executed with
> the max available parallele capacity and the second will "starve" on
> parllelism being executed sequentially. Is this correct?
> As a consequence to that, this also could mean that a query over a
> small table could take more advanatge (in parallel sense) than a scan
> on a larger table that was issued just a moment after (assuming both
> table can be scanned in parallel), right?

Precisely.  That is why you have "max_parallel_workers_per_gather"
to limit the number of parallel workers available to a single query.

Yours,
Laurenz Albe
-- 
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