[Bug] Precise union drops repeated keys in the SortShuffle path
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评估
调研方向
Read UnionFilterIterator.java, especially NoRepeatValueIterator.hasNext() and saveElement(), then trace how the reducer handles comparator-equal groups and EOF. Reproduce with 20,000 keys occurring twice and check behavior across flush boundaries and repeated probes. Done means each matching name group emits one representative while preserving the intended representative/value rule.
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描述
Bug Type
Logic / missing results in precise union queries.
Environment
Development source reviewed in PR #3395, Java 17. Reproduced with the production iterators and SortShuffle in a standalone JVM; no running Store cluster was used.
Expected and actual behavior
Once UnionFilterIterator has more than 10,000 map entries, it selects the SortShuffle/reducer path. NoRepeatValueIterator omits an entire comparator-equal group when that group occurs more than once, rather than returning one representative.
A probe with 20,000 keys, each read twice returns 0 rows; expected 20,000 rows, one per key. Each occurrence is a newly allocated BackendColumn with identical name/value bytes for that key.
The same query shape should retain each overlapping key once. MapUnionIterator already does so on the alternate small-query path.
Reproduction
- Generate keys as four-byte integer arrays and two occurrences per key, with fresh BackendColumn objects and arrays for every row.
- Populate an IntersectionWrapper from the first pass and call
proc(); all keys in this dataset are repeated candidates. - Construct UnionFilterIterator over an independent second pass, using the production BackendColumn serializer and name comparator.
- Consume normally with
while (iterator.hasNext()) { iterator.next(); count++; }. The observed count is zero.
The 10,000 threshold selects the union's sorting branch. SortShuffle has its own higher physical-file threshold, so this defect can occur before it writes an actual spill file; its file-backed path uses the same reducer.
Cause and focused follow-up
saveElement() preserves repeated candidates for sorting, but NoRepeatValueIterator.hasNext() skips groups where its duplicate counter is positive, including the final group. Fix reduction to emit a representative once for each matching name group, and cover repeated keys across flush boundaries, repeated probes, and EOF. Preserve the intended representative/value rule.
Sources: UnionFilterIterator and reducer, MapUnion alternate query path.
This is independent of the in-memory map-key defect tracked in https://github.com/apache/hugegraph/issues/3396 and predates the changed iterator-state logic in PR #3395. Keep it as a separate correctness fix rather than expanding that PR.
Issue search performed for NoRepeatValueIterator and UnionFilterIterator; no matching existing issue was found.
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