code: def upgrade():
      try:
          op.alter_column(
              "clusters",
              "changed_on",
              existing_type=sa.DATETIME(),
              nullable=True,
          )
          op.alter_column(
              "clusters",
              "created_on",
              existing_type=sa.DATETIME(),
              nullable=True,
          )
          op.drop_constraint(None, "columns", type_="foreignkey")
          op.drop_constraint(None, "columns", type_="foreignkey")
          op.drop_column("columns", "created_on")
          op.drop_column("columns", "created_by_fk")
          op.drop_column("columns", "changed_on")
          op.drop_column("columns", "changed_by_fk")
          op.alter_column(
              "css_templates",
              "changed_on",
              existing_type=sa.DATETIME(),
              nullable=True,
          )
          op.alter_column(
              "css_templates",
              "created_on",
              existing_type=sa.DATETIME(),
              nullable=True,
          )
          op.alter_column(
              "dashboards",
              "changed_on",
              existing_type=sa.DATETIME(),
              nullable=True,
          )
          op.alter_column(
              "dashboards",
              "created_on",
              existing_type=sa.DATETIME(),
              nullable=True,
          )
          op.create_unique_constraint(None, "dashboards", ["slug"])
          op.alter_column(
              "datasources",
              "changed_by_fk",
              existing_type=sa.INTEGER(),
              nullable=True,
          )
          op.alter_column(
              "datasources",
              "changed_on",
              existing_type=sa.DATETIME(),
              nullable=True,
          )
          op.alter_column(
              "datasources",
              "created_by_fk",
              existing_type=sa.INTEGER(),
              nullable=True,
          )
          op.alter_column(
              "datasources",
              "created_on",
              existing_type=sa.DATETIME(),
              nullable=True,
          )
          op.alter_column(
              "dbs",
              "changed_on",
              existing_type=sa.DATETIME(),
              nullable=True,
          )
          op.alter_column(
              "dbs",
              "created_on",
              existing_type=sa.DATETIME(),
              nullable=True,
          )
          op.alter_column(
              "slices",
              "changed_on",
              existing_type=sa.DATETIME(),
              nullable=True,
          )
          op.alter_column(
              "slices",
              "created_on",
              existing_type=sa.DATETIME(),
              nullable=True,
          )
          op.alter_column(
              "sql_metrics",
              "changed_on",
              existing_type=sa.DATETIME(),
              nullable=True,
          )
          op.alter_column(
              "sql_metrics",
              "created_on",
              existing_type=sa.DATETIME(),
              nullable=True,
          )
          op.alter_column(
              "table_columns",
              "changed_on",
              existing_type=sa.DATETIME(),
              nullable=True,
          )
          op.alter_column(
              "table_columns",
              "created_on",
              existing_type=sa.DATETIME(),
              nullable=True,
          )
          op.alter_column(
              "tables",
              "changed_on",
              existing_type=sa.DATETIME(),
              nullable=True,
          )
          op.alter_column(
              "tables",
              "created_on",
              existing_type=sa.DATETIME(),
              nullable=True,
          )
          op.alter_column(
              "url",
              "changed_on",
              existing_type=sa.DATETIME(),
              nullable=True,
          )
          op.alter_column(
              "url",
              "created_on",
              existing_type=sa.DATETIME(),
              nullable=True,
          )
      except Exception:
          pass
  
imports: import sqlalchemy
  from alembic import op
label_desc: Use image node_id 1 for calling a global function with example usage: upgrade() without return types