How to add an auto-incrementing primary key to an existing table, in PostgreSQL?

Question

I have a table with existing data. Is there a way to add a primary key without deleting and re-creating the table?

Answer 1

I landed here because I was looking for something like that too. In my case, I was copying the data from a set of staging tables with many columns into one table while also assigning row ids to the target table. Here is a variant of the above approaches that I used. I added the serial column at the end of my target table. That way I don't have to have a placeholder for it in the Insert statement. Then a simple select * into the target table auto populated this column. Here are the two SQL statements that I used on PostgreSQL 9.6.4.

ALTER TABLE target ADD COLUMN some_column SERIAL;
INSERT INTO target SELECT * from source;