Re: why not parallel seq scan for slow functions

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.  Also, even if inheritance is used, we might still be the
topmost scan/join target.

-- 
Robert Haas
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