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Expose Parquet Data Page dictionary index provenance to downstream readers

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Valutazione

Difficoltà
5/5
Tempo stimato
Più di una settimana
Idoneità per principianti
30/100
Tipo di issue
Funzionalità
Chiarezza
Da chiarire
Stato di attività
Attiva
Stack tecnologico
rust
Ambito
data-engineering, databases

Direzione di ricerca

Start by reviewing ColumnReader::read_records, DictIndexDecoder, and the private parquet::arrow::decoder and experimental encodings::rle paths. Compare the proposed callback and value-section decoder against the V1/V2 framing, null-count, fallback, truncation, and dictionary-ID requirements, including the fix from #10725. Done means maintainers agree on a supported public seam with a focused implementation and validation scope.

Scritto dal modello di indicizzazione a partire dal testo della issue.

Descrizione

Problem

A physical Parquet Page inspector needs the encoded dictionary entry ID consumed by each non-null value position of one RLE_DICTIONARY Data Page. This is different from the occurrence ordinal and from returning an Arrow DictionaryArray: decoded values can repeat while their occurrence ordinals advance, for example ordinals 0, 1, 2 consuming dictionary IDs 2, 2, 0.

ColumnReader::read_records returns repetition/definition levels and already materialized values, but does not expose the consumed dictionary IDs. DictIndexDecoder would decode the values section, but parquet::arrow::decoder is private (including on current main). The lower-level encodings::rle route is available only via the experimental feature, which carries no stability guarantee. This was verified against parquet 58.4.0 with downstream imports failing E0603. We are looking for a supported seam rather than making all decoder internals public.

Related #9010 asks to inspect dictionary contents in the async reader; this request is for per-occurrence IDs in a selected Data Page.

Possible interface

Would maintainers prefer either:

  1. an opt-in Page/column decode callback that pairs each level position with the optional dictionary ID and decoded value; or
  2. a public, validated value-section dictionary-index decoder with an explicit count contract, documented for composition with levels from ColumnReader?

The second route would let an inspector split the already decompressed selected Page into V1/V2 level and value sections, then align returned IDs only to positions whose definition level equals the schema maximum. A Page can fall back to PLAIN; those positions must have no dictionary ID. The API should permit bounded, opt-in use without adding per-value work to ordinary reads.

Correctness boundary

  • V1 and V2 level-section framing differ; a caller must isolate the values section before decoding indices.
  • The number of IDs must equal the number of non-null physical values, including for nested repetition/definition levels and V2 num_nulls.
  • Reject empty/truncated streams, invalid bit widths, and negative or out-of-range dictionary IDs. Current main has the empty-section/bit-width fix from #10725; an exposed seam should retain it.
  • No dictionary ID can be inferred by searching a decoded value in the dictionary, nor from its occurrence ordinal.
  • A PLAIN fallback Page must report no dictionary ID even when an earlier Page in the Column Chunk used a dictionary.

Could you advise which public interface fits parquet-rs ownership and stability expectations? We can contribute a focused implementation after the seam is agreed.

Lingua principale
Rust
Stelle
3.6k
Fork
1.3k
Merge medio
3g 10h
PR unite (30g)
148

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