vrozov commented on issue #1244: DRILL-6373: Refactor Result Set Loader for Union, List support URL: https://github.com/apache/drill/pull/1244#issuecomment-406811356 @paul-rogers The change to `NullableVarCharVector` that was part of your PR exposed a bug that is part of the existing code. Please see `AbstractMapVector.java:48`. The problem is that a newly created map vector references a newly created deep copy(clone) of the passed `MaterializedField` while newly created child vectors of the map vector references children of **another** instance of the `MaterializeField`. If that **other** instance is completely immutable it would not cause any problem (except that now there is no reason to create deep copy), but with your changes there was an attempt to mutate from different threads the **other** instance as that **other** instance is used to create multiple outgoing vectors. I hope that this explains what I mean when referring to **existing** bug.
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