Re: why not parallel seq scan for slow functions

On Mon, Nov 6, 2017 at 9:57 PM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>> 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.

Oh, wow. That's surprising.

>> 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.

--
Robert Haas
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