code: def downgrade():
      """Unapply Make XCom primary key columns non-nullable"""
      conn = op.get_bind()
      with op.batch_alter_table("xcom") as bop:
          # regardless of what the model defined, the `key` and `execution_date`
          # columns were always non-nullable for mysql, sqlite and postgres, so leave them alone
  
          if conn.dialect.name == "mssql":
              bop.drop_constraint("pk_xcom", type_="primary")
              # execution_date and key wasn't nullable in the other databases
              bop.alter_column(
                  "key", type_=StringID(length=512), nullable=True
              )
              bop.alter_column(
                  "execution_date", type_=TIMESTAMP, nullable=True
              )
  
imports: from __future__ import annotations
  from alembic import op
  from airflow.migrations.db_types import TIMESTAMP, StringID
label_desc: Use image node_id 2 for calling a global function with example usage: downgrade() without return types