It hasn't got anything to do with that. Creating an index on a join means that possibly hundreds of indexes need to be updated every time a you make a write - b

It hasn't got anything to do with that. Creating an index on a join means that possibly hundreds of indexes need to be updated every time a you make a write - but that is where appengine would actually shine since those writes can be parallelized to hundreds of machines. It could easily make postgres/mysql/oracle redundant.