postgres convert varchar to uuid column type code example

Example: postgres change column type to uuid

--  Ensure the UUID extension is installed.
CREATE EXTENSION IF NOT EXISTS "uuid-ossp";

--  Dropping and recreating the default column value is required because
--  the default INT value is not compatible with the new column type.
ALTER TABLE table_to_alter ALTER COLUMN table_id DROP DEFAULT, 
ALTER COLUMN table_id SET DATA TYPE UUID USING (uuid_generate_v4()), 
ALTER COLUMN table_id SET DEFAULT uuid_generate_v4();