apache/druid · error · IllegalArgumentException

Expected a long or a SpectatorHistogramMap, but received

Error message

Expected a long or a SpectatorHistogramMap, but received [%s] of type [%s]

What it means

SpectatorHistogramAggregator.aggregate received a selector value that is neither a Number nor a SpectatorHistogram. The aggregator expects either a plain long/numeric value or a histogram object produced by the matching column serializer; any other object (e.g. a String, Map, or a histogram from an incompatible serializer) is rejected, indicating a column-type/aggregator mismatch in the query spec.

Solutions

1. Check the query's aggregator spec: the spectatorHistogram aggregator must target a column serialized as SpectatorHistogram or numeric values.
2. Verify the input source's column type matches (e.g. not a string or complex type from another extension).
3. Inspect the logged object's type to identify which serializer produced the unexpected value.
4. Fix the query or the ingestion spec so the column's serialization matches the aggregator's expectations.

When it happens

Trigger: Thrown at extensions-contrib/spectator-histogram/src/main/java/org/apache/druid/spectator/histogram/SpectatorHistogramAggregator.java:65 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of apache/druid@9b90983fd2 (2026-09-07). Data as JSON: /api/errors/98e2aa7b40fe07ab. Report an issue: GitHub.

Appendix: source

Thrown at extensions-contrib/spectator-histogram/src/main/java/org/apache/druid/spectator/histogram/SpectatorHistogramAggregator.java:65

  @Override
  public void aggregate()
  {
    Object obj = selector.getObject();
    if (obj == null) {
      return;
    }
    if (obj instanceof SpectatorHistogram) {
      SpectatorHistogram other = (SpectatorHistogram) obj;
      synchronized (this) {
        counts.merge(other);
      }
    } else if (obj instanceof Number) {
      synchronized (this) {
        counts.insert((Number) obj);
      }
    } else {
      throw new IAE(
          "Expected a long or a SpectatorHistogramMap, but received [%s] of type [%s]",
          obj,
          obj.getClass()
      );
    }
  }

  @Nullable
  @Override
  public synchronized Object get()
  {
    return counts.isEmpty() ? null : counts;
  }

  @Override
  public synchronized float getFloat()
  {
    return counts.getSum();

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