UNION ALL takes forever with NULL dataset

问题

On a query like:

select col_A from t1
UNION ALL
select col_A from t2

were select col_A from t2 return zero rows, Oracle 11G is taking forever to run. If t2 returns at least 1 row, this query is at least 10 times faster. Any thoughts?

来源：https://stackoverflow.com/questions/45105606/union-all-takes-forever-with-null-dataset