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
