From: | Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> |
---|---|
To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
Cc: | David Rowley <david(dot)rowley(at)2ndquadrant(dot)com>, Robert Haas <robertmhaas(at)gmail(dot)com>, "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: WIP: Upper planner pathification |
Date: | 2016-03-07 04:35:41 |
Message-ID: | CAA4eK1+sULn6s4057jNQ5xpz4g0BtFdhXG49zCpo6DMcve263g@mail.gmail.com |
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On Mon, Mar 7, 2016 at 9:13 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>
> Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> writes:
> >>>> Is there some reason why hash and nestloop are safe but merge isn't?
>
> > To make hash and nestloop work in parallel queries, we just push those
> > nodes below gather node. Refer code
> > paths match_unsorted_outer()->consider_parallel_nestloop()
> > and hash_inner_and_outer()->try_partial_hashjoin_path().
>
> AFAICS, those are about generating partial paths, which is a completely
> different thing from whether a regular path is parallel-safe or not.
>
Okay, but the main point I wanted to convey is that I think setting
parallel_degree = 0 in mergejoin path is not necessarily a copy-paste
error. If you see the other path generation code like
create_index_path(), create_samplescan_path(), etc. there we set
parallel_degree to zero even though the parallel-safety is determined based
on reloptinfo. And I don't see any use of setting parallel_degree for path
which can't be pushed beneath gather (aka can be executed by multiple
workers).
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com
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