As you probably know PostgreSQL 10 introduced IDENTITY columns. They are pretty much like serial columns from the first sight. To learn about differences one may refer to the perfect blog post by Peter Eisentraut.

So I was wondering what if I mix some serial with some identity. So I executed such simple query:

CREATE TABLE foo( id SERIAL PRIMARY KEY GENERATED BY DEFAULT AS IDENTITY );

Of course, I got an error:

ERROR: both default and identity specified for column "id" of table "foo" SQL state: 42601

“SQL state: 42601” stands for a syntax error. But the interesting thing is that SERIAL, as you know, converted to CREATE SEQUENCE and SET DEFAULT.

So now I know what exactly the reason of error. You cannot have both DEFAULT and IDENTITY for a column.