Re: pg_stat_statements issue with parallel maintenance (Was Re: WAL usage calculation patch)

On Thu, Apr 2, 2020 at 8:34 AM Peter Geoghegan <pg(at)bowt(dot)ie> wrote:
>
> On Wed, Apr 1, 2020 at 7:52 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> > Peter, Is this behavior expected?
> >
> > Let me summarize the situation so that it would be easier for Peter to
> > comment. Julien has noticed that parallel vacuum and parallel create
> > index doesn't seem to report correct values for buffer usage stats.
> > Sawada-San wrote a patch to fix the problem for both the cases. We
> > expect that 'total_read_blks' as reported in pg_stat_statements should
> > give the same value for parallel and non-parallel operations. We see
> > that is true for parallel vacuum and previously we have the same
> > observation for the parallel query. Now, for parallel create index
> > this doesn't seem to be true as test results by Dilip show that. We
> > have two possibilities here (a) there is some bug in Sawada-San's
> > patch or (b) this is expected behavior for parallel create index.
> > What do you think?
>
> nbtree CREATE INDEX doesn't even go through the buffer manager.

Thanks for clarifying. So IIUC, it will not go through the buffer
manager for the index pages, but for the heap pages, it will still go
through the buffer manager.

> The
> difference that Dilip showed is probably due to extra catalog accesses
> in the two parallel workers -- pg_amproc lookups, and the like. Those
> are rather small differences, overall.

> Can Dilip demonstrate the the "extra" buffer accesses are
> proportionate to the number of workers launched in some constant,
> predictable way?

Okay, I will test this.

--
Regards,
Dilip Kumar
EnterpriseDB: http://www.enterprisedb.com