CONVERT_FROM() json format fails as below if the underlying Filter produces 0 rows:
If the conversion is applied as UTF8 format, the same query succeeds:
The reason for this is the special handling in the ProjectRecordBatch for JSON. The output schema is not known for this until the run time and the ComplexWriter in the Project relies on seeing the input data to determine the output schema - this could be a MapVector or ListVector etc.
If the input data has 0 rows due to a filter condition, we should at least produce a default output schema, e.g an empty MapVector ? Need to decide a good default. Note that the CONVERT_FROM(x, 'json') could occur on 2 branches of a UNION-ALL and if one input is empty while the other side is not, it may still cause incompatibility.