apache/druid · error · ISE

Got a [ ] which isn't a

Error message

Got a [%s] which isn't a %s

What it means

TopNQueryRunnerFactory's per-segment run() asserts that the incoming query is a TopNQuery before delegating to the TopN query engine. It throws IllegalStateException when any other query type reaches the TopN runner, indicating the query was routed to the wrong runner factory.

Solutions

1. Pass a TopNQuery (built with TopNQueryBuilder) into the runner
2. In tests, build the QueryPlus with new QueryPlus<>(topNQuery, ...) using a real or mocked TopNQuery
3. Fix factory registration so TopNQueryRunnerFactory only receives TopNQuery instances
4. instanceof-check query type before constructing runner chains

Example fix

// before
QueryPlus<Result<TopNResultValue>> input = new QueryPlus<>(groupByExample, null);
QueryRunner<Result<TopNResultValue>> r = factory.createRunner(segment);
r.run(input, new HashMap<>());
// after
TopNQuery topNExample = new TopNQueryBuilder().dataSource(ds).intervals(q).dimension(dim).metric(m).threshold(10).aggregators(aggs).build();
QueryPlus<Result<TopNResultValue>> input = new QueryPlus<>(topNExample, null);
QueryRunner<Result<TopNResultValue>> r = factory.createRunner(segment);
r.run(input, new HashMap<>());

if (!(queryPlus.getQuery() instanceof TopNQuery)) {
  throw new IllegalArgumentException("TopN runner requires TopNQuery, got " + queryPlus.getQuery().getClass());
}

static boolean isTopNQuery(QueryPlus<?> input) {
  return input.getQuery() instanceof TopNQuery;
}

try {
  segmentRunner.run(queryPlus, responseContext).toList();
} catch (IllegalStateException e) {
  if (e.getMessage().contains("which isn't a")) {
    // reroute via QueryRunnerFactoryConglomerate using the query's actual class
  }
  throw e;
}

When it happens

Trigger: Invoking TopNQueryRunnerFactory.SegmentTopNQueryRunner.run() with a QueryPlus holding a non-TopN query; calling it directly in unit tests (as seen in callers like testMultipleRunsThrowException, testCancel*) with a wrong-typed or mock QueryPlus; misrouted queries from a broker/historical where the conglomerate resolved to the TopN factory.

Common situations: Hand-written test harnesses constructing QueryPlus objects with the wrong query type; custom QueryRunner chains that bypass QueryRunnerFactoryConglomerate type dispatch; serialization/deserialization producing a base Query implementation.

Understand the failure class

Background: "This is a bug, please report it": internal invariant violations, unreachable panics, and SNH errors explained — this error's family across 47 libraries.

Related errors

• Can only handle [ ], got [ ]
• AggregatorFactoryNotMergeableException
• Expected expression with just one binding
• Expected expression with just one binding
• Number of segment descriptors does not equal number of…

AI-assisted analysis of apache/druid@9b90983fd2 (2026-09-07). Data as JSON: /api/errors/97f15876efd2b605. Report an issue: GitHub.

Appendix: source

Thrown at processing/src/main/java/org/apache/druid/query/topn/TopNQueryRunnerFactory.java:74

    this.computationBufferPool = computationBufferPool;
    this.toolchest = toolchest;
    this.queryWatcher = queryWatcher;
  }

  @Override
  public QueryRunner<Result<TopNResultValue>> createRunner(final Segment segment)
  {
    final TopNQueryEngine queryEngine = new TopNQueryEngine(computationBufferPool);
    return new QueryRunner<>()
    {
      @Override
      public Sequence<Result<TopNResultValue>> run(
          QueryPlus<Result<TopNResultValue>> input,
          ResponseContext responseContext
      )
      {
        if (!(input.getQuery() instanceof TopNQuery)) {
          throw new ISE("Got a [%s] which isn't a %s", input.getClass(), TopNQuery.class);
        }

        TopNQuery query = (TopNQuery) input.getQuery();
        return queryEngine.query(
            query,
            segment,
            (TopNQueryMetrics) input.getQueryMetrics()
        );
      }
    };

  }

  @Override
  public QueryRunner<Result<TopNResultValue>> mergeRunners(
      QueryProcessingPool queryProcessingPool,
      Iterable<QueryRunner<Result<TopNResultValue>>> queryRunners
  )

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