Re: What is the best way to create Primary Key on a large table in Postgresql 9.5?

You will have to write/create a brand new table file.  That will take time.  I suspect that using some combination of views and triggers, and object renaming, will allow you to put together a custom scheme to do this with minimal impact on production (though you cannot avoid the I/O, you can maybe spread it out).

A lot depends on the exact usage pattern of the table in question.

David J.