Hyukjin Kwon resolved SPARK-23173.
       Resolution: Fixed
         Assignee: Michał Świtakowski
    Fix Version/s: 2.4.0

Fixed in https://github.com/apache/spark/pull/20694

> from_json can produce nulls for fields which are marked as non-nullable
> -----------------------------------------------------------------------
>                 Key: SPARK-23173
>                 URL: https://issues.apache.org/jira/browse/SPARK-23173
>             Project: Spark
>          Issue Type: Bug
>          Components: SQL
>    Affects Versions: 2.2.1
>            Reporter: Herman van Hovell
>            Assignee: Michał Świtakowski
>            Priority: Major
>              Labels: release-notes
>             Fix For: 2.3.1, 2.4.0
> The {{from_json}} function uses a schema to convert a string into a Spark SQL 
> struct. This schema can contain non-nullable fields. The underlying 
> {{JsonToStructs}} expression does not check if a resulting struct respects 
> the nullability of the schema. This leads to very weird problems in consuming 
> expressions. In our case parquet writing would produce an illegal parquet 
> file.
> There are roughly solutions here:
>  # Assume that each field in schema passed to {{from_json}} is nullable, and 
> ignore the nullability information set in the passed schema.
>  # Validate the object during runtime, and fail execution if the data is null 
> where we are not expecting this.
> I currently am slightly in favor of option 1, since this is the more 
> performant option and a lot easier to do.
> WDYT? cc [~rxin] [~marmbrus] [~hyukjin.kwon] [~brkyvz]

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