Github user paul-rogers commented on the issue:
https://github.com/apache/drill/pull/594
We have three cases for nulls:
1. See a typed field, followed by nulls. Here, we know the type, just mark
the current field as null.
2. Run of nulls at start of file, followed by a non-null value within the
same record batch. We can transform the null field into a typed field. That is
what this fix tries to do.
3. Run of nulls in one record batch, followed by a non-null value in the
next batch. This must trigger a schema change. The question is, does it trigger
based on adding a new field, or assuming a type in the first batch, then
changing the type in the second?
The code presumably handles the first case. Case 3 is beyond the scope of
this fix. So, the question is, how best to handle the second case? Is the
present fix sufficient?
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