bujjibabukatta opened a new pull request, #17844:
URL: https://github.com/apache/iceberg/pull/17844
### Summary
Proposes one direction for #17463 (open investigation issue, no
consensus yet): let ORC surface struct-level null counts it already
computes, and let V4 content stats store them.
### Root Cause
- **ORC**: `ColumnStatistics[]` includes real null-count data for
struct columns, but `StatsColumnsVisitor.record()` only ever added
children's field IDs — the struct's own ID was discarded.
- **V4 stats**: `fieldStatsStruct()` excluded all nested types
(struct, map, list) uniformly via `isNestedType()`, so structs had
no way to store counts even when a writer could supply them.
Read side is unaffected — `InclusiveMetricsEvaluator` already does a
generic field-ID lookup with no leaf/struct distinction.
### Fix
- `OrcMetrics.java` — `StatsColumnsVisitor.record()` now also adds
the struct's own field ID via `ORCSchemaUtil.icebergID(record)`.
Bounds unaffected (struct `ColumnStatistics` matches no
`instanceof` branch in `fromOrcMin`/`fromOrcMax`).
- `StatsUtil.java` — `fieldStatsStruct()` returns a reduced
`value_count`/`null_value_count`-only struct for `StructType`
instead of `null`. Map/list still excluded as before.
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