BUG #17724: All autovacuum workers operate on 1 db at a time

The following bug has been logged on the website:

Bug reference:      17724
Logged by:          Peter Plachta
Email address:      pplachta@gmail.com
PostgreSQL version: Unsupported/Unknown
Operating system:   Windows (Azure)
Description:

Hi all

I have observed this on PG 9.6 and 10 and could not find a bug indicating it
was fixed.

Issue is that autovacuum workers appear bound to a certain DB, even if there
is more urgent (wraparound) work to do elsewhere.

In our case specifically we have multiple DBs, one has a large table that
for various reasons takes a long time (many hours) to vacuum. During this
time other autovacuum workers are idle while our other DBs which do frequent
insert/delete accumulate significant bloat which ends up affecting query
time.

Ideally the idle autovacuum workers would just proceed to the next DB.