Re: Parallel Seq Scan

On Mon, Apr 20, 2015 at 10:08 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>
> On Tue, Apr 7, 2015 at 11:58 PM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>
wrote:
> > One disadvantage of retaining parallel-paths could be that it can
> > increase the number of combinations planner might need to evaluate
> > during planning (in particular during join path evaluation) unless we
> > do some special handling to avoid evaluation of such combinations.
>
> Yes, that's true. But the overhead might not be very much. In the
> common case, many baserels and joinrels will have no parallel paths
> because the non-parallel paths is known to be better anyway. Also, if
> parallelism does seem to be winning, we're probably planning a query
> that involves accessing a fair amount of data,

Am I understanding right that by above you mean to say that retain the
parallel and non-parallel path only if parallel-path wins over non-parallel
path?

If yes, then I am able to understand the advantage of retaining both
parallel and non-parallel paths, else could you explain some more
why you think it is advantageous to retain parallel-path even when it
losses to serial path in the beginning?

With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com