max_parallel_workers can't actually be set?

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: pgsql-hackers(at)lists(dot)postgresql(dot)org
Subject: max_parallel_workers can't actually be set?
Date: 2019-08-17 17:41:18
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Try this:
alter system set max_parallel_workers = 20;
and restart the system.

max_parallel_workers is still 8, according to both SHOW and
pg_controldata, nor can you launch more than 8 workers.

Even odder, if you just do

regression=# set max_parallel_workers = 200;
regression=# show max_parallel_workers;
(1 row)

which should certainly not happen for a PGC_POSTMASTER parameter.

We seem to have an awful lot of mechanism that's concerned with
adjustments of max_parallel_workers, for something that apparently
might as well be a compile-time #define ... so I assume it's supposed
to be changeable at restart and somebody broke it. But it's not
working as I'd expect in any branch from 10 onwards.

regards, tom lane


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