Github user paul-rogers commented on the issue:
https://github.com/apache/drill/pull/914
Added final development stage to this PR. This is a minor implementation
tweak. The result set loader internals for maps held redundant map vectors. In
the final vector container, each map must be represented by a map (or repeated
map) vector. But, because the result set loader handles projection and
overflow, the set of columns that a map writer works with is a superset of
those that appear in the output container. For this reason, there turns out to
be no reason to maintain these "redundant" map vectors.
For repeated map vectors, we must maintain the offset vector. The code
already did this; we just strip off the enclosing repeated map vector.
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