On Tue, Sep 1, 2020 at 05:26:50PM +0000, PG Doc comments form wrote: > The following documentation comment has been logged on the website: > > Page: https://www.postgresql.org/docs/12/runtime-config-query.html > Description: > > I recently have an observation that the combination of random_page_cost and > parallel_leader_participation can cause a performance regression. However, > based on the official website, it seems that these two configurations seem > not to have a very straight relationship. > I run the PostgreSQL 11 on Ubuntu 18.04. When I create a database with > about 16MB and run a select query by pgbench. If I set the random_page_cost > as 4 and parallel_leader_participation is on, the average latency of select > query is about 72 ms for five clients. However, if we change the > random_page_cost to 1 or turn off the parallel_leader_participation, the > average latency is only 54ms. However, If I manually run the query on a > client, changing the value of random_page_cost doesn't change the > performance > I check the workload and find that if the random_page_cost is 4 and > parallel_leader_participation is on, query planer would tend to choose Merge > gather join for select clause, which would require more memory to do > sorting. If the random_page_cost is 1 and parallel_leader_participation is > on, the query plan would choose nest loop, which requires less memory. I > guess this might be the root cause of performance regression. > If my guess is true, I would suggest mentioning that the combination of > random_page_cost and paraller_leader_participation would cause the query to > use more memory in the official document
There are a lot of interactions between settings, and mentioning all of them would be impossible. I suggest you ask this on our performance email list, and then you can get some ideas on the caues of this. -- Bruce Momjian <br...@momjian.us> https://momjian.us EnterpriseDB https://enterprisedb.com The usefulness of a cup is in its emptiness, Bruce Lee
