GH-47995: [C++][Parquet] Fix empty string min/max statistics being lost during merge #48717
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…ing lost during merge Prior to this change, CleanStatistic() for ByteArray rejected statistics where ptr == nullptr. However, empty strings can have ptr == nullptr with len == 0, causing valid statistics to be discarded when the minimum value is an empty string. The fix introduces a sentinel pointer (kNoValueSentinel) distinct from nullptr to mark "no value" in ByteArray statistics. This allows CleanStatistic to distinguish between "no min/max computed" (sentinel) and "min/max is empty string" (nullptr with len=0). FLBA is unchanged since it has fixed length and no "empty" concept.
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Rationale for this change
Fixes #47995
When merging ByteArray statistics, empty string min/max values were incorrectly discarded. This happened because
CleanStatistic()rejected statistics whereptr == nullptr, but empty strings can legitimately haveptr == nullptrwithlen == 0.What changes are included in this PR?
Introduces a sentinel pointer (
kNoValueSentinel) distinct fromnullptrto mark "no value" in ByteArray statistics. This allowsCleanStatisticto distinguish between:FLBA is unchanged since it has fixed length and no "empty" concept.
Are these changes tested?
Yes. Added comprehensive tests covering all combinations of:
Are there any user-facing changes?
No API changes. This is a bug fix that preserves empty string statistics correctly during merge operations.