Dynamic data model, locks and performance

Hello people of the Postgresql world!

I am wondering if Postgresql would a great choice for my database needs.

I would like to create a db with dynamic data model. I would like to be able
to add tables and columns to existing tables while other queries are
running.

Will Postresql be able to manage the locks correctly? Am I pushing
Postgresql too far by wanting to do something like this? If so, what do you
suggest instead?

Also, I would like to be able to have millions of tables in my database. As
I know, Postgresql uses one file per table. So is this problem related to
performance about mapping correctly the files with the file system? Is
Posgresql able to handle this kind of charge?

-- 
A+

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Pierre