Postgresql install fails version 9.6

[Dean Taylor]

Message:
Unable to install packages postgresql-9.6,postgresql-client-9.6,postgresql-client-common,postgresql-common,postgresql-contrib-9.6

Deploy instruction:
juju deploy postgresql pg-a --config='version=9.6'

Deployed to OpenStack server and LXD image with same result
Using Ubuntu 20.04 client
Juju version: 2.8.0-focal-amd64

Verified that the LXD image can install the snap version of postgresql96
LXD image deployed: Ubuntu Bionic
Apt packages support:
postgresql 10

[Dean Taylor (codemerc)]

Investigation shows that the charm does not install the The PostgreSQL Global Development Group (PGDG) APT repository of PostgreSQL packages for Debian and Ubuntu located at http://apt.postgresql.org/pub/repos/apt/.
Since the charm utilities Apt for the installation I am assuming this was the original source of the binaries.

[Dean Taylor (codemerc)]

Found the configuration option "pgdg" (boolean) Enable the PostgreSQL Global Development Group APT repository.

Would suggest that this is not a bug so much as a feature request or enhancement.

If a PostgreSQL version is explicitly stated and it is not the version provided by the main Ubuntu archive then pgdg option is automatically enabled.

[Dean Taylor (codemerc)]

Would also suggest that an entry in the documentation under "version" referencing the pgdg configuration option as a possible prerequisite.

[Stuart Bishop (stub)]

At a minimum, update description in config.yaml.

Consider enabling implicitly when the version is not the default for the series. Maybe this would be surprising, or maybe it will be required always for backports of useful tools like pgrepack or repmgr.