DataColumn
DataColumn is an essential part for low-level serialization. It represents a column that has actual data.
For simple records that contain atomic types (int, string etc.) schema only consists of DataColumns.
Here is a sample logical representation of DataColumn class:
Field is a schema field that defines this column. You can obtain this field from a schema you define.
DefinedData is raw data that is defined by Field's type. If your field is nullable, DefinedData represents non-nullable values. On the other hand, Data represents data as-is, including nulls. If you are reading DataColumn and need to access the data, Data is your field. If you need to access data as it's stored in parquet file, use DefinedData. The names are chosen mostly due to backward compatibility reasons.
Going further, if you need to access repetition and definition levels as they are stored in parquet file, you can use the corresponding DefinitionLevels and RepetitionLevels fields.
Creating DataColumn
There are two public constuctors available (see above diagram). For convenience and backward compatibility, the second constuctor accepts DataField and two parameters:
datais data to write, including nulls if the field is nullable. DataColumn will decompose the data array intoDefinitionLevelsandDefinedDataon construction.repetitionLevelsare only required if a field is a part of a nested type.
The first constructor is more granular and allows you to specify all three parts when constructing a colum.