> create table t(x int); insert into t(x) values (1), (2), (3);
> create sequence s;
> alter table t add column y int default nextval('s');
pq: nextval(): cannot backfill such sequence operation

> set experimental_serial_normalization=sql_sequence; -- for pg compat
>  alter table t add column y serial;
pq: nextval(): cannot backfill such sequence operation

> begin; 
  insert into t(x) values (4); 
  alter table t add column y int default nextval('d'); 
  commit;
pq: transaction committed but schema change aborted with error: (42P15): nextval(): cannot backfill such sequence operation
HINT: Some of the non-DDL statements may have committed successfully, but some of the DDL statement(s) failed.
Manual inspection may be required to determine the actual state of the database.
--
See: https://github.com/cockroachdb/cockroach/issues/42061
> select * from t;
  x
+---+
  1
  2
  3
  4
(4 rows)