code: def downgrade():
      op.drop_column("tables", "schema_perm")
      op.drop_column("datasources", "schema_perm")
      op.drop_column("slices", "schema_perm")
  
file_path: superset/superset/migrations/versions/2019-09-11_21-49_5afa9079866a_serialize_schema_permissions_py.py
imports: from alembic import op
  from sqlalchemy import Column, ForeignKey, Integer, String
  from sqlalchemy.ext.declarative import declarative_base
  from sqlalchemy.orm import relationship
  from superset import db
label_desc: Use image node_id 2 for calling a global function with example usage: downgrade() without return types