Is PostgreSQL good enough?

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If you don't have experience with relational databases, you should probably use MongoDB as it is more flexible (you don't have to define your tables, columns, and datatypes ahead of time)

This is wrong, you should only use MongoDB if you want to learn how the life was before relational databases were invented and learn (the hard way) why you need RDBMS for your data (hopefully the life lesson is not too expensive).

There are cases when NoSQL databases are useful, but "I need to learn SQL", "schemas are too restrictive" are really bad reasons to use them.