Re: why not parallel seq scan for slow functions

On Wed, Nov 8, 2017 at 6:48 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> On Wed, Nov 8, 2017 at 7:26 AM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>> We do want to generate it later when there isn't inheritance involved,
>> but only if there is a single rel involved (simple_rel_array_size
>> <=2). The rule is something like this, we will generate the gather
>> paths at this stage only if there are more than two rels involved and
>> there isn't inheritance involved.
>
> Why is that the correct rule?
>
> Sorry if I'm being dense here. I would have thought we'd want to skip
> it for the topmost scan/join rel regardless of the presence or absence
> of inheritance.
>

I think I understood your concern after some offlist discussion and it
is primarily due to the inheritance related check which can skip the
generation of gather paths when it shouldn't. So what might fit
better here is a straight check on the number of base rels such that
allow generating gather path in set_rel_pathlist, if there are
multiple baserels involved. I have used all_baserels which I think
will work better for this purpose.

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