from sqlalchemy import cast
from sqlalchemy import Column
from sqlalchemy import DateTime
from sqlalchemy import Integer
from sqlalchemy import MetaData
from sqlalchemy import String
from sqlalchemy import Table
from sqlalchemy import TypeDecorator


class StringDate(TypeDecorator):
    impl = DateTime

    def column_expression(self, col):
        return cast(col, String)


meta = MetaData()
t = Table(
    "t",
    meta,
    Column("Id", Integer, primary_key=True),
    Column("x", StringDate),
)

t_history = Table(
    "t_history",
    meta,
    Column("Id", Integer),
    Column("x", StringDate),
)

print(t.select().union_all(t_history.select()))

SELECT t."Id", CAST(t.x AS VARCHAR) AS x 
FROM t UNION ALL SELECT t_history."Id", t_history.x 
FROM t_history

print(t.select().union_all(t_history.select()))