What database is most suitable for use in the Messenger?

Question

Database

I, for educational purposes, am trying to completely write my messenger for android.
Obviously, for this I need to use some kind of database on the server side, but searching the Internet and asking questions to some teachers did not clarify the situation, because some say that NOSQl databases are most suitable for the messenger, others that the best and most versatile option is PostgreSQL.
I also came across one of the implementations of the rethinkdb messenger, but when I started looking for information about it, I found an article that this database was abandoned by the creators as an unsuccessful project. So, if anyone knows, can you tell us more about it?

Answer 1

As my teacher told me in my first year at the university, there is no need to do premature optimization. Then the same quote in other forms was told to me by different companies. What is the point - when in practice you see the inability of the current database to cope with the load, then you will understand its limitations, and you can find a database without these limitations.
For educational purposes, the database is not as important as the organization of storage of this data. For example, if there is no index in the table, the search speed will be slower, and so on and so forth.
PS And now it is direct specifically on a choice of a DB. What do you mean by "Messenger"? Where is the ER diagram of your database? How do you want to deal with the scaling problem when it comes up?