On Thu, Nov 16, 2017 at 10:23 AM, Ants Aasma <ants.aasma@eesti.ee> wrote:
> For the Gather Merge driven by Parallel Index Scan case it seems to me
> that the correct queue size is one that can store two index pages
> worth of tuples. Additional space will always help buffer any
> performance variations, but there should be a step function somewhere
> around 1+1/n_workers pages. I wonder if the queue could be dynamically
> sized based on the driving scan. With some limits of course as parent
> nodes to the parallel index scan can increase the row count by
> arbitrary amounts.
Currently, Gather Merge can store 100 tuples + as much more stuff as
fits in a 64kB queue. That should already be more than 2 index pages,
I would think, although admittedly I haven't tested.
--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company
