Its long been known that Oracle is a very bad choice for Moodle installs and we would recommend against people using that (unless really no choice in the matter).
However, we do not have many docs explicitly mentioning that. Furthermore its sort of 'unwritten' recommendation that postgres if a very good choice for large installs. But people new to the community do not know this.
I think we need some clearer docs explaining our recommendations for database/web-server platforms. This might be a bit of a 'political' issue to give recommendations - but actually I think we should. People in the community for some time know that postgres and apache are solid dependable choices - we should make that clearer to newcomers.