Re: Observations in Parallel Append

On Sun, Dec 24, 2017 at 12:06 PM, Robert Haas <robertmhaas@gmail.com> wrote:
> On Fri, Dec 22, 2017 at 6:18 AM, Amit Kapila <amit.kapila16@gmail.com> wrote:
>
>> Also, don't we need to use parallel_divisor for partial paths instead
>> of non-partial paths as those will be actually distributed among
>> workers?
>
> Uh, that seems backwards to me.  We're trying to estimate the average
> number of rows per worker.
>

Okay, but is it appropriate to use the parallel_divisor?  The
parallel_divisor means the contribution of all the workers (+
leader_contribution) whereas for non-partial paths there will be
always only the subset of workers which will operate on them.
Consider a case with one non-partial subpath and five partial subpaths
with six as parallel_divisor, now the current code will try to divide
the rows of non-partial subpath with respect to six workers.  However,
in reality, there will always be one worker which will execute that
path.

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