Parquet reader: fall back to positional field IDs when a file has no field IDs and no name mapping #1046

Background

#1044 adds name mapping support to the Parquet reader. When a Parquet file has no field IDs and no name mapping is provided, the reader returns InvalidSchema, which matches the current C++ Avro reader (avro_reader.cc).

The Java Parquet reader handles this case differently. As pointed out in #1044, ReadConf falls back to addFallbackIds and assigns positional IDs to top-level columns, so such files can still be read.

Proposal

When the Parquet file schema has no field IDs and no name mapping is provided, assign fallback IDs by top-level column position, as Java does, instead of returning an error.

Open questions:

• Should the Avro reader get the same fallback for consistency?
• Should the fallback be opt-in or always enabled?