On 2015-07-29 AM 11:02, Kouhei Kaigai wrote:
>>
>> ...
>> synchronous Append path vs. parallel asynchronous Append with Funnel
>> (below/above?) it. I guess the asynchronous version would always be
>> cheaper. So, even if we end up with non-parallel sub-plans do we still add
>> a Funnel to make Append asynchronous? Am I missing something?
>>
> I expect Funnel itself will get Append capability but run sub-plans in
> background workers, to simplify path constructions. So, if Funnel with
> multiple sub-plans have cheaper cost than Append, it will replace the
> AppendPath by FunnelPath.
>
> Regarding to the cost estimation, I don't think parallel version is always
> cheaper than traditional Append, because of the cost to launch background
> workers. It increases startup cost to process the relation, thus, if upper
> node prefers small startup cost (like Limit), traditional Append still has
> advantages.
>
Right, I almost forgot about the start-up cost.
Thanks,
Amit