Re: [DESIGN] ParallelAppend

On Fri, Nov 20, 2015 at 12:45 AM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> Okay, but I think that's not what I am talking about. I am talking about
> below code in cost_seqscan:
>
> - if (nworkers > 0)
>
> - run_cost = run_cost / (nworkers + 0.5);
>
> + if (path->parallel_degree > 0)
>
> + run_cost = run_cost / (path->parallel_degree + 0.5);
>
>
> It will consider 50% of master backends effort for scan of each child
> relation,
> does that look correct to you? Wouldn't 50% of master backends effort be
> considered to scan all the child relations?

In the code you originally wrote, you were adding 1 there rather than
0.5. That meant you were expecting the leader to do as much work as
each of its workers, which is clearly a bad estimate, because the
leader also has to do the work of gathering tuples from the workers.
0.5 might not be the right value, but it's surely better than 1.

--
Robert Haas
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