GAE DataStore vs Google Cloud SQL for Enterprise Management Systems

From the Cloud SQL FAQ:

Should I use Google Cloud SQL or the App Engine Datastore?

This depends on the requirements of the application. Datastore provides NoSQL key-value > storage that is highly scalable, but does not support the complex queries offered by a SQL database. Cloud SQL supports complex queries and ACID transactions, but this means the database acts as a ‘fixed pipe’ and performance is less scalable. Many applications use both types of storage.

If you need a lot of writes (~XXX per/s) to db entity w/ distributed keys, that's where the Google App Engine datastore really shine.

If you need support for complex and random user crafted queries, that's where Google Cloud SQL is more convenient.

What is scare me more in GAE datastore is index number limitation. For example if you need search by some field or sorting - you need +1 index. Totally you can have 200 indexes. If you have entity with 10 searchable fields and you can sort by any field - there will be about 100 combunations. So you need 100 indexes. I have developed few small projects for gae - and this is success stories. But when big one come - this is not for gae.

About cache - you can do it with gae, but they distributed cache works very slow. I prefer to create private single instance of permanent backend with RESTfull API that holds cached values in memory. Frontend instances call this API to get/set values.

Maybe it is posible to build complex system with gae, but this will be a set of small applications/services.