apache/druid · error · org.apache.druid.java.util.common.IAE

Comparing histograms is not supported

Error message

Comparing histograms is not supported

What it means

Like the CDF post-aggregator, the histogram output (double[]) has no defined total ordering, so DoublesSketchToHistogramPostAggregator.getComparator() always throws IllegalArgumentException('Comparing histograms is not supported'). Reaching it means the engine tried to sort or compare results by the histogram column.

Solutions

1. Sort by a scalar post-aggregator (e.g. a quantile) on the same sketch instead of the histogram.
2. Remove the sorting reference to the histogram column entirely.
3. Fetch unsorted rows and sort client-side if needed.

Example fix

// before
{"queryType":"groupBy","limitSpec":{"columns":[{"dimension":"hist"}]}} // throws
// after
{"queryType":"groupBy","limitSpec":{"columns":[{"dimension":"q50"}]}} // scalar post-agg

if (sortColumns.stream().anyMatch(histogramPostAggNames::contains)) { throw new IllegalArgumentException("Cannot sort by histogram post-aggregator output"); }

try { druidClient.run(query); } catch (IllegalArgumentException e) { if (e.getMessage().contains("Comparing histograms")) { /* remove sorting reference and rerun */ } }

When it happens

Trigger: Using the histogram post-aggregator's output as an ORDER BY key, topN metric, or limit-spec sorting column in a native query.

Common situations: ORDER BY on a histogram column in SQL, topN with the histogram as metric, or groupBy limit spec sorting on the histogram.

Related errors

• Comparing arrays of quantiles is not supported
• Comparing histograms is not supported
• Comparing sketch summaries is not supported
• Comparing arrays of estimate values is not supported
• Comparing arrays of estimates and error bounds is not…

AI-assisted analysis of apache/druid@9b90983fd2 (2026-09-07). Data as JSON: /api/errors/92c1d22c79f386a4. Report an issue: GitHub.

Appendix: source

Thrown at extensions-core/datasketches/src/main/java/org/apache/druid/query/aggregation/datasketches/quantiles/DoublesSketchToHistogramPostAggregator.java:155

  @JsonProperty
  @JsonInclude(JsonInclude.Include.NON_NULL)
  public double[] getSplitPoints()
  {
    return splitPoints;
  }

  @JsonProperty
  @JsonInclude(JsonInclude.Include.NON_NULL)
  public Integer getNumBins()
  {
    return numBins;
  }

  @Override
  public Comparator<double[]> getComparator()
  {
    throw new IAE("Comparing histograms is not supported");
  }

  @Override
  public Set<String> getDependentFields()
  {
    return field.getDependentFields();
  }

  @Override
  public String toString()
  {
    return getClass().getSimpleName() + "{" +
        "name='" + name + '\'' +
        ", field=" + field +
        ", splitPoints=" + Arrays.toString(splitPoints) +
        ", numBins=" + numBins +
        "}";
  }

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