I should have been clearer - when I said “external PostgreSQL database”, I meant not on the same server as XWiki.
That’s why I picked xwiki-tomcat9-common because to pick xwiki-tomcat9-pgsql results in installing a local copy of PostgreSQL, which is not what I want.
In the meantime, I’ve managed to get the missing hibernate.cfg.xml from the WAR file … but I see no reason why it isn’t included in the -common DEB.
Each database package generate a pre configured hibernate file and if xwiki-tomcat9-common was also providing one it would cause a conflict. Maybe a better solution would be something like a new xwiki-tomcat9-external package which expose a form to generate a proper hibernate.cfg.xml.
Another alternative solution would be to include, say, hibernate.cfg.xml.example or something similarly named within xwiki-tomcat9-common? That would avoid the need for another package whilst providing at least easier access to a file that could be used?
Anyway, at least I better understand the situation now, so thanks.