Github user yhuai commented on the pull request:
https://github.com/apache/spark/pull/1346#issuecomment-48561811
Hi @ueshin, @marmbrus and I discussed about it. We think it is not
semantically clear what a null means when it appears in the key or value field
(considering a null is used to indicate a missing data value). So, we decided
that `key` and `value` in a `MapType` should not contain any null value and we
will not introduce `containsNull` to `MapType`. Does it make sense?
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