text: ALTER TABLE public_dataset_versions
      ADD COLUMN schema_version TEXT NOT NULL DEFAULT '';
  
  update public_dataset_versions set schema_version='1.0' WHERE created_at < '2019-09-11';
  update public_dataset_versions set schema_version='2.0' WHERE created_at < '2019-09-20' and created_at > '2019-09-11';
  update public_dataset_versions set schema_version='3.0' WHERE created_at > '2019-09-20';
  
  ALTER TABLE public_dataset_versions
      ALTER COLUMN schema_version DROP DEFAULT;