Re: Parallel Append implementation

On Thu, Sep 14, 2017 at 8:30 PM, Amit Khandekar <amitdkhan(dot)pg(at)gmail(dot)com> wrote:
> On 11 September 2017 at 18:55, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>>>
>>
>> How? See, if you have four partial subpaths and two non-partial
>> subpaths, then for parallel-aware append it considers all six paths in
>> parallel path whereas for non-parallel-aware append it will consider
>> just four paths and that too with sub-optimal strategy. Can you
>> please try to give me some example so that it will be clear.
>
> Suppose 4 appendrel children have costs for their cheapest partial (p)
> and non-partial paths (np) as shown below :
>
> p1=5000 np1=100
> p2=200 np2=1000
> p3=80 np3=2000
> p4=3000 np4=50
>
> Here, following two Append paths will be generated :
>
> 1. a parallel-aware Append path with subpaths :
> np1, p2, p3, np4
>
> 2. Partial (i.e. non-parallel-aware) Append path with all partial subpaths:
> p1,p2,p3,p4
>
> Now, one thing we can do above is : Make the path#2 parallel-aware as
> well; so both Append paths would be parallel-aware.
>

Yes, we can do that and that is what I think is probably better. So,
the question remains that in which case non-parallel-aware partial
append will be required? Basically, it is not clear to me why after
having parallel-aware partial append we need non-parallel-aware
version? Are you keeping it for the sake of backward-compatibility or
something like for cases if someone has disabled parallel append with
the help of new guc in this patch?

>
> So above, what I am saying is, we can't tell which of the paths #1 and
> #2 are cheaper until we calculate total cost. I didn't understand what
> did you mean by "non-parallel-aware append will consider only the
> partial subpaths and that too with sub-optimal strategy" in the above
> example. I guess, you were considering a different scenario than the
> above one.
>

Yes, something different, but I think you can ignore that as we can
discuss the guts of my point based on the example given by you above.

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