Re: Proposal : Parallel Merge Join

On Mon, Feb 27, 2017 at 10:27 AM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> Okay, but in that case don't you think it is better to consider the
> parallel safety of cheapest_total_inner only when we don't find any
> cheap parallel_safe innerpath by reducing the sort keys?

Well, we can do that but suppose cheapest_total_inner is not parallel
safe and we do not get any parallel safe path which is cheaper than
cheapest_total_inner, then we just end up making the merge join path
with the cheapest parallel safe path but we might have missed some of
the paths whose pathkey is covering more ordered keys. Still, it's
hard to argue what it better because we can always say that if we try
only cheapest parallel safe path we will generate fewer paths.

--
Regards,
Dilip Kumar
EnterpriseDB: http://www.enterprisedb.com