Yes, but we should use it to achieve lazy behavior, otherwise it will be impact on performance

Problem there that you are using fetch="select" on joins.

Alexander, Animal.hbm.xml - it is different mapping, which uses joined-subclass instead of subclass with discriminator + join.

We can get the correct results using joined-subclass (see Animal.hbm.xml mapping file) but in this case there are a lot of left outer joins to all the subclasses (that could impact the performance):

select
children1_3_.Pregnant as col_0_0_
from
t_animal animal0_
inner join
t_animal children1_
on animal0_.Id=children1_.ParentId
left outer join
t_reptile children1_1_
on children1_.Id=children1_1_.Id
left outer join
t_lizard children1_2_
on children1_.Id=children1_2_.Id
left outer join
t_mammal children1_3_
on children1_.Id=children1_3_.Id
left outer join
t_dog children1_4_
on children1_.Id=children1_4_.Id
left outer join
t_cat children1_5_
on children1_.Id=children1_5_.Id
where
case
when children1_2_.Id is not null then 2
when children1_4_.Id is not null then 4
when children1_5_.Id is not null then 5
when children1_1_.Id is not null then 1
when children1_3_.Id is not null then 3
when children1_.Id is not null then 0
end=3

Could it be possible to generate such SQL-query in case of using subclasses with discriminators if it is impossible to get SQL like (see the issue description):

select m._Pregnant, ...
from t_animal animal0_
inner join t_animal children1_ on animal0_.Id=children1_.ParentId
inner join t_mammal m
on children1_.Id=m.Id
where children1_.ANIMAL_TYPE='MAMMAL'

Anatoly, is your last attachement differents to one in the patch?