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fix: reject duplicate Parquet field names before decoding #5786

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fix: reject duplicate Parquet field names before decoding #5786

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79abecd
fix: reject duplicate Parquet field names before decoding
ErikBPF Sep 9, 2026
fc6b3d9
docs: remove issue reference from duplicate-field limitation
ErikBPF Sep 9, 2026
b696cc3
fix: scope duplicate checks to projected roots
ErikBPF Sep 12, 2026
07dc72b
test: cover pruned duplicate Parquet fields
ErikBPF Sep 13, 2026
38b8fd1
fix: validate only safely decoded Parquet fields
ErikBPF Sep 13, 2026
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9 changes: 9 additions & 0 deletions docs/source/user-guide/latest/compatibility/scans.md
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Expand Up @@ -62,6 +62,15 @@ The following limitation may produce incorrect results without falling back to S

The following limitations raise an error at scan time rather than falling back to Spark:

- Byte-identical sibling field names in selected top-level columns, including inside structs,
arrays, and maps. Comet rejects these before decoding to prevent row multiplication and decoder
synchronization errors. Unselected columns and safely pruned nested fields are skipped.
Reads requiring a full-subtree cast still validate that subtree. Files with embedded Arrow
schema hints and Variant scans conservatively validate selected subtrees in full; field-ID
reads validate the entire file schema. The check applies in both case-sensitivity modes;
names in separate groups do not collide. Disable Comet for the query to use Spark's duplicate-name
resolution with an explicit read schema. Spark-compatible resolution is tracked in
[#5884](https://github.com/apache/datafusion-comet/issues/5884).
- Invalid UTF-8 bytes in `STRING` columns. Spark permits arbitrary byte sequences in a `STRING`
column (for example from `CAST(X'C1' AS STRING)`), but Comet's native execution path is built on
Arrow, whose string type is strictly UTF-8. Reading a Parquet file whose `STRING` column contains
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