Re: Delay locking partitions during query execution

On Fri, 4 Jan 2019 at 02:40, Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com> wrote:
> I'm a bit confused, because I can't reproduce any such speedup. I've
> used the attached script that varies the number of partitions (which
> worked quite nicely in the INSERT thread), but I'm getting results like
> this:
>
> partitions 0 100 1000 10000
> --------------------------------------------
> master 49 1214 186 11
> patched 53 1225 187 11
>
> So I don't see any significant speedup, for some reason :-(
>
> Before I start digging into this, is there something that needs to be
> done to enable it?

Thanks for looking at this.

One thing I seem to quite often forget to mention is that I was running with:

plan_cache_mode = force_generic_plan
max_parallel_workers_per_gather = 0;

Without changing plan_cache_mode then the planner would likely never
favour a generic plan since it will not appear to be very efficient
due to the lack of consideration to the costing of run-time partition
pruning.

Also, then with a generic plan, the planner will likely want to build
a parallel plan since it sees up to 10k partitions that need to be
scanned. max_parallel_workers_per_gather = 0 puts it right.

(Ideally, the planner would cost run-time pruning, but it's not quite
so simple for RANGE partitions with non-equality operators. Likely
we'll want to fix that one day, but that's not for here)

--
David Rowley http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services