Re: [HACKERS] Parallel Hash take II

Hi,

On 2017-07-26 20:12:56 +1200, Thomas Munro wrote:
> Here is a new version of my parallel-aware hash join patchset.

Yay!

Working on reviewing this. Will send separate emails for individual
patch reviews.


> 2.  Simplified costing.  There is now just one control knob
> "parallel_synchronization_cost", which I charge for each time the
> participants will wait for each other at a barrier, to be set high
> enough to dissuade the planner from using Parallel Hash for tiny hash
> tables that would be faster in a parallel-oblivious hash join.
> Earlier ideas about modelling the cost of shared memory access didn't
> work out.

Hm. You say, "didn't work out" - could you expand a bit on that? I'm
quite doubtful that justaccounting for barriers will be good enough.


> I'll report on performance separately.

Looking forward to that ;)

Greetings,

Andres Freund