Re: Adding description about the random_page_cost and parallel_leader_participation

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 <bruce(at)momjian(dot)us> https://momjian.us
EnterpriseDB https://enterprisedb.com

The usefulness of a cup is in its emptiness, Bruce Lee