Postgres table contains 1 column, and that column is PK. How to insert new record?
Doing a insert .. select max() ... is a bad idea to begin with as it doesn't work correctly in a multi user environment. Plus it would get the sequence behind the serial column out of sync with the already generated values.

What you are looking for is: 

insert into bar (bar_id) values (default);


Another option would be explicitly call nextval() when inserting.

insert into bar (bar_id) values (nextval('bar_bar_id_seq'));