API: StructProjection returns a null Projection object when the nested struct is null

[stevenzwu]

Today, if the nested struct is null, StructProjection returns a nested StructProjection object wraps the null struct value. It is probably simpler if a null projection object is returned instead. Then the projection classes like (StructProjection or Flink RowDataProjection) don't need to handle null wrapped struct.

See more context in #7493 (comment).

@rdblue @aokolnychyi @RussellSpitzer @Reo-LEI

[stevenzwu]

if we have a schema like this

Schema schema =
        new Schema(
            Types.NestedField.required(0, "id", Types.LongType.get()),
            Types.NestedField.optional(
                3,
                "location",
                Types.StructType.of(
                    Types.NestedField.required(1, "lat", Types.FloatType.get()),
                    Types.NestedField.required(2, "long", Types.FloatType.get()))));

when the nested location struct is null

GenericRowData rowDataNullStruct = GenericRowData.of(1L, null);

the StructProjection or Flink RowDataProjection would create a projection object wrapping a null struct value. I was proposing that it should just return a null projection object directly.

[stevenzwu]

before: StructProjection(1, StructProjection(null))
after: StructProjection(1, null)

[stevenzwu]

then the Projection class doesn't have to deal with null wrapped value. e.g.

 public <T> T get(int pos, Class<T> javaClass) {
    if (struct == null) {
      // Return a null struct when projecting a nested required field from an optional struct.
      // See more details in issue #2738.
      return null;
    }

it also mirroring the original struct more faithfully.

Struct(1, null) -> StructProjection(1, null)    |   instead of StructProjection(1, StructProjection(null))

[github-actions]

This issue has been automatically marked as stale because it has been open for 180 days with no activity. It will be closed in next 14 days if no further activity occurs. To permanently prevent this issue from being considered stale, add the label 'not-stale', but commenting on the issue is preferred when possible.

[github-actions]

This issue has been closed because it has not received any activity in the last 14 days since being marked as 'stale'

[nastra]

@stevenzwu does this still apply? If so, then we should re-open and add the not-stale tag so that the issue doesn't get automatically closed