blackmwk commented on code in PR #2307:
URL: https://github.com/apache/iceberg-rust/pull/2307#discussion_r3026144097
##########
crates/iceberg/src/arrow/reader.rs:
##########
@@ -1111,14 +1111,24 @@ fn build_field_id_map(parquet_schema:
&SchemaDescriptor) -> Result<Option<HashMa
}
/// Build a fallback field ID map for Parquet files without embedded field IDs.
-/// Position-based (1, 2, 3, ...) for compatibility with iceberg-java
migrations.
+///
+/// Must use top-level field positions (not leaf column positions) to stay
consistent
+/// with `add_fallback_field_ids_to_arrow_schema`, which assigns ordinal IDs to
+/// top-level Arrow fields. Using leaf positions instead would produce wrong
indices
+/// when nested types (struct/list/map) expand into multiple leaf columns.
+///
+/// Matches iceberg-java's ParquetSchemaUtil.addFallbackIds().
fn build_fallback_field_id_map(parquet_schema: &SchemaDescriptor) ->
HashMap<i32, usize> {
Review Comment:
Got it, thanks for the explaination! Hopefully we could refactor it someday
to make the logic more similar to java.
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