Re: why not parallel seq scan for slow functions

On Wed, Nov 8, 2017 at 2:51 AM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> On Mon, Nov 6, 2017 at 9:57 PM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
>>> Also, even if inheritance is used, we might still be the
>>> topmost scan/join target.
>>
>> Sure, but in that case, it won't generate the gather path here (due to
>> this part of check "!root->append_rel_list"). I am not sure whether I
>> have understood the second part of your question, so if my answer
>> appears inadequate, then can you provide more details on what you are
>> concerned about?
>
> I don't know why the question of why root->append_rel_list is empty is
> the relevant thing here for deciding whether to generate gather paths
> at this point.
>

This is required to prohibit generating gather path for top rel in
case of inheritence (Append node) at this place (we want to generate
it later when scan/join target is available). For such a case, the
reloptkind will be RELOPT_BASEREL and simple_rel_array_size will be
greater than two as it includes child rels as well. So, the check for
root->append_rel_list will prohibit generating gather path for such a
rel. Now, for all the child rels of Append, the reloptkind will be
RELOPT_OTHER_MEMBER_REL, so it won't generate gather path here because
of the first part of check (rel->reloptkind == RELOPT_BASEREL).

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