If you add a default to a new column, existing rows in the table gain the default value in the new column.
For more information about defaults, see CREATE TABLE statement.
If you specify CASCADE, then the column drop should additionally drop other schema objects which have become invalid.
The schema objects which can cause a DROP COLUMN RESTRICT to be rejected include: views, triggers, primary key constraints, foreign key constraints, unique key constraints, check constraints, and column privileges.
To work around such conflicts, use the RESTART WITH syntax to specify the next value that will be generated for the identity column.