Thanks, this is a duplicate for #1269 which got closed without further investigation.
But, ok, so we got double confirmation that this gets 10 connection as a default.
Now the question is whether 10 is just the minimal number (for instance enforced by the PG library), or whether this doesn't respect configs at all (also for config value > 10).

I tried around a bit and it seems the config is not respected at all. When I set it to 20, there were still 10 connections.

I also tried overriding this directly in a lua plugin (by copying the default postgres plugin and adding size = 1):

require "auth/auth_commons"
-- import shared database query logic
require "auth/postgres_cockroach_commons"

function auth_on_register(reg)
    return auth_on_register_common(postgres, reg)
end

pool = "auth_postgres"
config = {
    pool_id = pool,
    size = 1
}

postgres.ensure_pool(config)

This also does not change the behaviour.

One interesting effect I saw though is, that my plugin is now run 10 times. I have a log statement at top which normally has been printed only once but now 10 times. Not sure if that connected to the issue.

Is there any known workaround or do you have any idea how I can investigate this further?

Looks like the 10 comes from here: https://github.com/vernemq/vernemq/blob/master/apps/vmq_diversity/src/vmq_diversity_sup.erl#L77

I can't really read erlang, but it seems that this file is the culprit.

Technically, it's in the function clause for postgres:

But your totally right. The ProviderConfig there comes in over the vmq_diversity_postgres.erl, but size simply gets reset to default.

Do you want to have a go at it? Can fix it myself, but low prio ;)

Note to self: we could upgrade to and test epgsql 4.4.0, instead of 4.3.0
https://github.com/epgsql/epgsql/tags

Did a PR here. Seems to work in my environment, but not sure if this has any unintended side effects.