GH-51779: [C++][IPC] Read a DictionaryEncoding without indexType as int32 indices - #51807
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…e as int32 indices
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CaptainAni187
marked this pull request as ready for review
October 5, 2026 23:54
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Rationale for this change
Schema.fbsallowsDictionaryEncoding.indexTypeto be omitted: "If this field is null, the indices must be signed int32." The C++ IPC reader required it instead, so C++ and PyArrow failed to read such a stream withIOError: Unexpected null field DictionaryEncoding.indexType in flatbuffer-encoded metadata.What changes are included in this PR?
FieldFromFlatbufferusesint32()as the index type whenindexTypeis absent, instead of failing the null check.Are these changes tested?
Yes.
TestMessageInternal.DictionaryEncodingWithoutIndexTypebuilds a schema message whose dictionary-encoded field has noindexTypeand checks thatReadSchemareturnsdictionary<values=string, indices=int32>. On main it fails with the error above. Thearrow-ipc-*tests pass locally.Are there any user-facing changes?
Streams that omit
indexTypecan now be read. Streams that include it are read as before.Was AI used for this PR?
In accordance to the AI generation guidelines, please disclose below whether and how AI was used in this PR.
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