Postgres is an open-source project and it has DocBook as default format for its documentation. At a first glance it looks like a tree of *.sgml files in the doc directory inside a repository.
There are several pre-defined convertion output formats, but unfortunately Emacs' native one is ignored.
Does it possible to get Postgres documentation as a postgres.info.gz file?
That's basically nothing more than a text conversion problem. I believe the right solution here would be to write an XSL that converts the XML in your SGML files to TeXinfo source code, but the next best thing:
pandoc
is a parser for different textual document file formats. It has a reader for Docbook and a writer for texinfo. That should get you started.