Observations

LSB equivalence between definition levels and bitmaps

When the maximum repetition level is 0 and the maximum definition level is 1, the RLE-encoded definition levels correspond exactly to Arrow's bitmap and can be memcopied without further transformations.

Nested parquet groups are deserialized recursively

Reading a parquet nested field is done by reading each primitive column sequentially, and build the nested struct recursively.

Rows of nested parquet groups are encoded in the repetition and definition levels. In arrow, they correspond to:

• list's offsets and validity

• struct's validity

The implementation in this module leverages this observation:

Nested parquet fields are initially recursed over to gather whether the type is a Struct or List, and whether it is required or optional, which we store in nested_info: Vec<Box<dyn Nested>>. Nested is a trait object that receives definition and repetition levels depending on the type and nullability of the nested item. We process the definition and repetition levels into nested_info.

When we finish a field, we recursively pop from nested_info as we build the StructArray or ListArray.

With this approach, the only difference vs flat is:

1. we do not leverage the bitmap optimization, and instead need to deserialize the repetition and definition levels to i32.

2. we deserialize definition levels twice, once to extend the values/nullability and one to extend nested_info.