Re: [HACKERS] Parallel tuplesort (for parallel B-Tree index creation)

On Thu, Jan 18, 2018 at 10:27 AM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> On Thu, Jan 18, 2018 at 1:14 PM, Peter Geoghegan <pg(at)bowt(dot)ie> wrote:
>> That seems pretty far fetched.
>
> I don't think it is, and there are plenty of other examples. All you
> need is a query plan that involves significant CPU work both below the
> Gather node and above the Gather node. It's not difficult to find
> plans like that; there are TPC-H queries that generate plans like
> that.

You need to have a very selective qual in the worker, that eliminates
most input (keeps the worker busy), and yet manages to keep the leader
busy rather than waiting on input from the gather.

>> But even if it wasn't, my position
>> would not change. This could happen only because the planner
>> determined that it was the cheapest plan when
>> parallel_leader_participation happened to be off. But clearly a
>> "degenerate parallel CREATE INDEX" will never be faster than a serial
>> CREATE INDEX, and there is a simple way to always avoid one. So why
>> not do so?
>
> That's an excellent argument for making parallel CREATE INDEX ignore
> parallel_leader_participation entirely.

I'm done making arguments about parallel_leader_participation. Tell me
what you want, and I'll do it.

--
Peter Geoghegan