Re: why not parallel seq scan for slow functions

On Mon, Nov 6, 2017 at 7:05 PM, Robert Haas <robertmhaas@gmail.com> wrote:
> On Mon, Nov 6, 2017 at 11:20 AM, Amit Kapila <amit.kapila16@gmail.com> wrote:
>> On Mon, Nov 6, 2017 at 3:51 AM, Robert Haas <robertmhaas@gmail.com> wrote:
>>> This looks like it's on the right track to me.  I hope Tom will look
>>> into it, but if he doesn't I may try to get it committed myself.
>>>
>>> -    if (rel->reloptkind == RELOPT_BASEREL)
>>> -        generate_gather_paths(root, rel);
>>> +    if (rel->reloptkind == RELOPT_BASEREL &&
>>> +        root->simple_rel_array_size > 2 &&
>>> +        !root->append_rel_list)
>>>
>>> This test doesn't look correct to me.  Actually, it doesn't look
>>> anywhere close to correct to me.  So, one of us is very confused...
>>> not sure whether it's you or me.
>>>
>> It is quite possible that I haven't got it right, but it shouldn't be
>> completely bogus as it stands the regression tests and some manual
>> verification.  Can you explain what is your concern about this test?
>
> Well, I suppose that test will fire for a baserel when the total
> number of baserels is at least 3 and there's no inheritance involved.
> But if there are 2 baserels, we're still not the topmost scan/join
> target.
>

No, if there are 2 baserels, then simple_rel_array_size will be 3.
The simple_rel_array_size is always the "number of relations" plus
"one".  See setup_simple_rel_arrays.

>  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?


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