[mdaniel]

I was just trying to get a sense for where the line of demarcation was in your mind. PG has "foreign data wrappers"[1] that allow one to treat external ... whatever ... as if it were a table or procedure within PG. Just stupid powerful, IMHO. It is FDW-specific whether "transaction" means anything to the foreign system, so that may break your mental model but could still get you very close (e.g. BEGIN; UPDATE s3 SET ...; ROLLBACK; may not do anything sensible)

https://github.com/turbot/steampipe#steampipe-plugins and https://steampipe.io/docs/steampipe_postgres/overview may be relevant, although watch out for Steampipe's license

https://github.com/topics/foreign-data-wrapper and https://github.com/topics/fdw are some other examples

1: https://www.postgresql.org/docs/17/fdwhandler.html (although strictly speaking that page is for _authoring_ FDW, not a tl;dr of the concept)