Just stumbled over this announcement: aws.amazon.com/de/blogs/opensource/want-...ight-like-babelfish/
tl,dr: if this succeeds, you will be able to use your app, written for communication with MS-SQLServer, and let it talk to PostgreSQL. Presumably, this will be getting "hot" sometime next year, but IMHO this is great news.
Regards
Karl (X# 2.14.0.4; Xide 1.33; W8.1/64 German)