optimizer implicitly assumed that if `a` in `a=b` is not NULL,
then it's safe to convert `a` to the type of `b` and search the
result in the index(b).

which is not always the case, as converting a non-null value
to a different type might produce NULL. And searching for NULL
in the index might find NULL there, so NULL will be equal to NULL,
making `a=b` behave as if it was `a<=>b`