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

Comparing arrays of quantiles is not supported

Error message

Comparing arrays of quantiles is not supported

What it means

DoublesSketchToQuantilesPostAggregator returns an array of quantile values (double[]). Since arrays of quantiles have no natural ordering, getComparator() throws IllegalArgumentException('Comparing arrays of quantiles is not supported') whenever the engine requests a comparator for this output.

Solutions

1. Order by a single scalar value instead (e.g. DoublesSketchToQuantilePostAggregator for one fraction).
2. Remove the sorting spec that references the quantiles post-aggregator.
3. Sort client-side after retrieving results.

Example fix

// before
{"orderByColumn":"quantiles"} // array output, throws
// after
{"orderByColumn":"median"} // quantilesDoublesSketchToQuantile post-agg (scalar)

if (orderByColumns.contains(quantilesPostAggName)) { throw new IllegalArgumentException("Cannot ORDER BY quantiles array output; use a scalar quantile post-agg"); }

try { druidClient.run(query); } catch (IllegalArgumentException e) { if (e.getMessage().contains("Comparing arrays of quantiles")) { /* replace sort key */ } }

When it happens

Trigger: Referencing the quantiles post-aggregator output in an ORDER BY clause, as a topN metric, or in a groupBy limitSpec sorting column.

Common situations: Trying to order query results by the quantiles array; copying a limitSpec that named the sketch column without realizing the post-agg output is an array.

Related errors

• Comparing histograms 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/d1bcefe209c76ab4. Report an issue: GitHub.

Appendix: source

Thrown at extensions-core/datasketches/src/main/java/org/apache/druid/query/aggregation/datasketches/quantiles/DoublesSketchToQuantilesPostAggregator.java:97

    return fractions;
  }

  @Override
  public Object compute(final Map<String, Object> combinedAggregators)
  {
    final DoublesSketch sketch = (DoublesSketch) field.compute(combinedAggregators);
    if (sketch.isEmpty()) {
      final double[] quantiles = new double[fractions.length];
      Arrays.fill(quantiles, Double.NaN);
      return quantiles;
    }
    return sketch.getQuantiles(fractions);
  }

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

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

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

View on GitHub (pinned to 9b90983fd2)