Re: Parallel Index Scans

On Mon, Oct 17, 2016 at 8:08 PM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> Create Index .... With (parallel_workers = 4);
>
> If above syntax looks sensible, then we might need to think what
> should be used for parallel index build. It seems to me that parallel
> tuple sort patch [1] proposed by Peter G. is using above syntax for
> getting the parallel workers input from user for parallel index
> builds.

Apparently you see a similar issue with other major database systems,
where similar storage parameter things are kind of "overloaded" like
this (they are used by both index creation, and by the optimizer in
considering whether it should use a parallel index scan). That can be
a kind of a gotcha for their users, but maybe it's still worth it. In
any case, the complaints I saw about that were from users who used
parallel CREATE INDEX with the equivalent of my parallel_workers index
storage parameter, and then unexpectedly found this also forced the
use of parallel index scan. Not the other way around.

Ideally, the parallel_workers storage parameter will rarely be
necessary because the optimizer will generally do the right thing in
all case.

--
Peter Geoghegan