How to overcome performance issues caused by the generated union SQL subclass in the Hibernate basement

I am using Hibernate support for the inheritance strategy TABLE_PER_CLASS. Functionality is wise; it works well. Whenever a polymorphic query is issued, Hibernate generates SQL containing "union all" for my two specific classes A and B. The generated SQL has the following format:

select C1, C2, C3 from (
    select C1, C2, C3 from ClassA
  union all
    select C1, C2, C3 from ClassB
)
where 
    C1 == <value>
order by C2
limit 100

The problem with this approach is suffering from very poor database performance. Given that column C1 is a common property of ClassA and ClassB (derived from an abstract parent), Hibernate can insert a where clause into both selections and significantly improve performance. For instance,

select C1, C2, C3 from ( 
    select C1, C2, C3 from ClassA where C1 == <value>
    union all
    select C1, C2, C3 from ClassB where C1 == <value>
)
order by C2
limit 100

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