apache/druid · error · IllegalArgumentException

Comparing arrays of quantiles is not supported

Error message

Comparing arrays of quantiles is not supported

What it means

KllDoublesSketchToQuantilesPostAggregator.getComparator always throws IllegalArgumentException because an array of quantile values has no defined comparison semantics. The method exists to satisfy the post-aggregator interface but intentionally rejects any ordering request.

Source

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

    return fractions;
  }

  @Override
  public Object compute(final Map<String, Object> combinedAggregators)
  {
    final KllDoublesSketch sketch = (KllDoublesSketch) 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)

Solutions

  1. Order by a specific scalar quantile instead: use kllDoublesSketchToQuantile (singular) with a single fraction
  2. Avoid ORDER BY on the quantiles array column
  3. Skip comparison-requiring paths for this post-aggregator in generic processing code

Example fix

// before
{"type":"kllDoublesSketchToQuantiles","name":"qs","field":f,"fractions":[0.5]} ... ORDER BY qs
// after
{"type":"kllDoublesSketchToQuantile","name":"median","field":f,"fraction":0.5} ... ORDER BY median
Defensive patterns

Strategy: validation

Validate before calling

if (postAgg instanceof KllDoublesSketchToQuantilesPostAggregator) {
  throw new IllegalArgumentException("Quantiles array output is not sortable; use kllDoublesSketchToQuantile");
}

Try / catch

try { cmp = postAgg.getComparator(); } catch (IllegalArgumentException e) { cmp = null; }

Prevention

When it happens

Trigger: Ordering or ranking on the output of kllDoublesSketchToQuantiles, e.g. ORDER BY on its result column, or framework code that calls getComparator() on all post-aggregators.

Common situations: Trying to sort results by quantile arrays; building generic UI sort controls over all post-aggregator columns; SQL queries that attempt to order on array-typed post-aggregator output.

Understand the failure class

Background: UnsupportedOperationException and "is not supported" errors: when a library deliberately refuses a call — this error's family across 30 libraries.

Related errors

AI-assisted analysis of apache/druid@9b90983fd2 (2026-09-07). Data as JSON: /api/errors/4848fb637a06b1d0. Report an issue: GitHub.