Re: Parallel Seq Scan
| От | Amit Kapila |
|---|---|
| Тема | Re: Parallel Seq Scan |
| Дата | |
| Msg-id | CAA4eK1LjUp-XGh3Qd1Q3kAGG=p_znQs=9E+F65pQk2iVtYLVPg@mail.gmail.com обсуждение исходный текст |
| Ответ на | Re: Parallel Seq Scan (Robert Haas <robertmhaas@gmail.com>) |
| Ответы |
Re: Parallel Seq Scan
|
| Список | pgsql-hackers |
On Mon, Apr 20, 2015 at 10:08 PM, Robert Haas <robertmhaas@gmail.com> wrote:
>
> On Tue, Apr 7, 2015 at 11:58 PM, Amit Kapila <amit.kapila16@gmail.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,
>
> On Tue, Apr 7, 2015 at 11:58 PM, Amit Kapila <amit.kapila16@gmail.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?
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