How would I select only one property of some objects?

If you look for the equivalent of something like SELECT DISTINCT date FROM posts your best bet is to findAll() posts, iterate over them and collect the data you need. That sounds slow and memory-eating?

Probably. Think about your use case, how would you handle this in a world without RDBMS'? Think about your data structure and try to find a way to model that efficiently. In other words - is that what you need?

The problem with thinking in terms of SQL is, we provide object persistence. If you receive single properties, you no longer deal with objects, and what good is a list of dates without relation to any posts?

GROUP BY and JOIN are other examples where the we-are-used-to-it problem clashes with the new paradigms. What do you get from two objects if you join them? How do you group objects like in SQL?

All that said, Lazy Loading will come back in FLOW3, this could help with the performance problem you expect in the loop-solution. And we'll try to find a way to do something like that what you need - if it makes sense. If that finds it's way into the codebase it could be something like the way JSR-283 describes table-like views on nodes.

How do JOINS work with the persistence layer?

See above answer for some background.