On Thu, Nov 16, 2017 at 10:23 AM, Ants Aasma <ants.aa...@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
