[martinald]

Yep, I agree there are cases where mongodb will perform better. However, many use cases also require joins and the other goodness that relations provide.

So really the use case for mongo etc is 'very high performance requirements' AND 'does not require relations'.

Many projects may be ok with just one of those. But very few require meet both of those constraints.

FWIW I've seen many cases which are sort of the opposite: great performance with mongodb, but then because of the lack of relations for a reporting feature (for example) performance completely plummets due to horrible hacks being done to query the data model that doesn't work with the schema, eventually requiring a rewrite to RDBMS. I would guess that this is much more common.