Re: parallel vacuum - few questions on docs, comments and code

On Tue, May 11, 2021 at 6:31 PM Justin Pryzby <pryzby(at)telsasoft(dot)com> wrote:
> > 4) IIUC, below comment says that even if PARALLEL 0 is specified with
> > VACUUM command, there are chances that the indexes are vacuumed in
> > parallel. Isn't it a bit unusual that a user specified 0 workers but
> > still the system is picking up parallelism? I'm sure this would have
> > been discussed, but I'm curious to know the reason.
> > * nrequested is the number of parallel workers that user requested. If
> > * nrequested is 0, we compute the parallel degree based on nindexes, that is
> > * the number of indexes that support parallel vacuum.
>
> No - nrequested is not actually the number of workers requested - it seems like
> a poor choice of name.
>
> This is the key part:
>
> src/include/commands/vacuum.h
> * The number of parallel vacuum workers. 0 by default which means choose
> * based on the number of indexes. -1 indicates parallel vacuum is
> * disabled.
> */
> int nworkers;
> } VacuumParams;

Thanks. The name "nworkers" looks fine to me after reading the comment
above it. And the parallelism will be chosen by default.
/* By default parallel vacuum is enabled */
params.nworkers = 0;

With Regards,
Bharath Rupireddy.
EnterpriseDB: http://www.enterprisedb.com