Fix GROUP BY DISTINCTCOUNT ClassCastException in mergeDataTablesOnly

[navina]

Background

BrokerReduceService exposes two reduce paths:

• reduceResult(...) — the normal path: merge per-server results and finalize.
• mergeDataTablesOnly(...) — merge-only: produce an intermediate DataTable that can be re-merged later via the normal reduce path. Used by callers that need to combine partial scatters without finalizing.

The merge-only path's contract is that aggregate values stay as intermediates end-to-end.

What's wrong

OSS's single-stage broker request handler auto-sets SERVER_RETURN_FINAL_RESULT=true when the routing table targets exactly one server (BaseSingleStageBrokerRequestHandler.java:844). That optimization causes servers to return finalized scalars (e.g. Integer for DISTINCTCOUNT, Long for DISTINCTCOUNTHLL) instead of intermediate sketches — which directly contradicts the merge-only contract: the input is already final-typed, not intermediate.

Two ways this manifested in GroupByDataTableReducer.mergeDataTablesOnly:

1. No guard for the single-server-final-result mode. When _queryContext.isServerReturnFinalResult() is true, the input DataTables hold final values, so merge-only cannot honor its contract. The previous code silently proceeded and produced a malformed intermediate.

2. getIndexedTable over-finalized internally. Even when servers correctly returned intermediates, the shared helper unconditionally called indexedTable.finish(true, true). For a GROUP BY query with an OBJECT-typed intermediate aggregate (DISTINCTCOUNT, DISTINCTCOUNTHLL, etc.), storeFinalResult=true:

   • mutated DataSchema._columnDataTypes from OBJECT to the final-result type in place (IndexedTable.java:170-174), and
   • replaced each row's value with extractFinalResult(value) (Set → Integer for DISTINCTCOUNT) at IndexedTable.java:195.

   The subsequent buildIntermediateDataTable read DataSchema's lazily-populated _storedColumnDataTypes (populated earlier from the pre-finalize OBJECT schema and never invalidated by finish), so the OBJECT branch fired and handed the now-Integer value into BaseDistinctAggregateAggregationFunction.serializeIntermediateResult(Set), which threw ClassCastException.

Fix

• Reject the single-server final-result mode explicitly in mergeDataTablesOnly: throw UnsupportedOperationException with a clear message naming SERVER_RETURN_FINAL_RESULT. Callers expecting an intermediate must either disable this option or use reduceResult instead.
• Move finish() out of getIndexedTable and let each caller pick the right mode:
  • reduceResult keeps finish(true, true) — downstream consumers expect final scalars.
  • mergeDataTablesOnly calls finish(true, false) so aggregate values stay as intermediates and round-trip through buildIntermediateDataTable correctly.

Tests

Adds testGroupByDistinctCountObjectRoundTrip in MergeDataTablesOnlyTest: servers emit intermediate OBJECT-encoded Sets, merge produces a re-injectable intermediate DataTable, and reducing that intermediate matches a direct reduce of the same per-server inputs.

Release Notes

Fixes a ClassCastException in mergeDataTablesOnly when merging GROUP BY results that include OBJECT-typed intermediate aggregates (e.g. DISTINCTCOUNT, DISTINCTCOUNTHLL). mergeDataTablesOnly now also throws UnsupportedOperationException up front when called against a query whose servers will return final results (e.g. the single-server SERVER_RETURN_FINAL_RESULT auto-set), instead of silently producing a malformed intermediate.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 64.77%. Comparing base (302783f) to head (27b3b99).

Additional details and impacted files

@@            Coverage Diff            @@
##             master   #18842   +/-   ##
=========================================
  Coverage     64.77%   64.77%           
  Complexity     1322     1322           
=========================================
  Files          3393     3393           
  Lines        211022   211023    +1     
  Branches      33135    33135           
=========================================
+ Hits         136687   136695    +8     
+ Misses        63322    63310   -12     
- Partials      11013    11018    +5     

Flag	Coverage Δ
custom-integration1	100.00% <ø> (ø)
integration	100.00% <ø> (ø)
integration1	100.00% <ø> (ø)
integration2	0.00% <ø> (ø)
java-21	64.77% <100.00%> (+<0.01%)	⬆️
temurin	64.77% <100.00%> (+<0.01%)	⬆️
unittests	64.77% <100.00%> (+<0.01%)	⬆️
unittests1	56.98% <100.00%> (-0.01%)	⬇️
unittests2	37.18% <0.00%> (+<0.01%)	⬆️

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