On Fri, 2019-12-20 at 13:03 -0500, Jeff Janes wrote: > The docs for parallel_tuple_cost are quite terse, as the reference section of > the docs usually are: > > "Sets the planner's estimate of the cost of transferring one tuple from a > parallel worker process to another process. The default is 0.1." > > Usually you can find more extensive discussion of such settings in informal > resources like blog posts or mailing lists, > but Googling the name I don't find much for this setting. Is there good > information out there somewhere? > > If you were take the doc description literally, then the default value seems > much too high, as it doesn't take > 10x the (default) cpu_tuple_cost to transfer a tuple up from a parallel > worker. On the other hand, you probably > don't want a query which consumes 8x the CPU resources just to finish only 5% > faster (on an otherwise idle server with 8 CPUs). > Maybe this Amdahl factor is what inspired the high default value?
Hmm. The parameter was introduced into the discussion here: https://www.postgresql.org/message-id/CAA4eK1L0dk9D3hARoAb84v2pGvUw4B5YoS4x18ORQREwR%2B1VCg%40mail.gmail.com and while the name was changed from "cpu_tuple_comm_cost" to "parallel_tuple_cost" later, the default value seems not to have been the subject of discussion. Yours, Laurenz Albe -- Cybertec | https://www.cybertec-postgresql.com
