如果A可以的话，您可以使用进行一次查询select_related。
from django.db import connection
q = A.objects.filter(NAME__istartswith='z').order_by('ORDER').select_related('b', 'c')
for obj in q:
obj = obj.b or obj.c or obj
print repr(obj), obj.__dict__ # (to prove the subclass-specific attributes exist)
print "query count:", len(connection.queries)
602392714
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