code: def downgrade():
      with op.batch_alter_table("dbs") as batch_op:
          batch_op.alter_column(
              "sqlalchemy_uri",
              existing_type=sa.VARCHAR(length=1024),
              nullable=True,
          )
  
imports: import sqlalchemy
  from alembic import op
label_desc: Use image node_id 2 for calling a global function with example usage: downgrade() without return types