Github user paul-rogers commented on the issue:
https://github.com/apache/drill/pull/594
Note that Drill *does* have a vector that can possibly used to represent a
run of nulls: the {{ZeroVector}}. Using this, we can:
* On the first record where we see a field, if that field is null, add a
{{ZeroVector}} to the record batch.
* On subsequent records, if the value is still null, do nothing.
* If the value is non-null, and the current vector is a {{ZeroVector}},
replace it with the proper Nullable vector, with all (0..i-1) values set to
null, and the ith value set to the current column.
* At end of batch, if any {{ZeroVector}}s remain, simply remove them so
that the column does not appear in the batch output.
The result of this is that we need not do two map lookups for a null value,
we just do one: the one to find the column value vector as we'd do for an int
or string.
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