apache/druid · error · IllegalArgumentException

Comparing histograms is not supported

Error message

Comparing histograms is not supported

What it means

The KLL sketch to CDF post-aggregator produces a cumulative distribution array of doubles, which has no meaningful ordering. Druid's post-aggregator interface requires getComparator() to be implemented; this class intentionally throws IAE because two histograms/CDFs cannot be compared. It surfaces only if a query tries to order, compare, or use this post-aggregator where a Comparator is required.

Source

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

    return ColumnType.DOUBLE_ARRAY;
  }

  @JsonProperty
  public PostAggregator getField()
  {
    return field;
  }

  @JsonProperty
  public float[] getSplitPoints()
  {
    return splitPoints;
  }

  @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) +
        "}";
  }

View on GitHub (pinned to 9b90983fd2)

Solutions

  1. Remove any ORDER BY / sort expression that references the CDF post-aggregator output; sort by a scalar column instead.
  2. If sorting is needed, compute a scalar value (e.g. a specific CDF rank via KLL_SKETCH_TO_QUANTILE) and order on that.
  3. Do not use this post-aggregator in contexts that require a Comparator (having-clause comparisons, ordered aggregations).

Example fix

// before
... ORDER BY "cdf_output" ASC

// after
... ORDER BY "myScalarColumn" ASC
Defensive patterns

Strategy: validation

Validate before calling

if (postAgg instanceof KllFloatsSketchToCDFPostAggregator && requiresComparator(querySpec)) {
  throw new IllegalArgumentException("KLL_SKETCH_TO_CDF output is not comparable; do not use in ORDER BY/having");
}

Prevention

When it happens

Trigger: Using KLL_SKETCH_TO_CDF as an ORDER BY expression, in a comparison, or anywhere Druid calls PostAggregator.getComparator() (e.g. order-by specs, having clauses referencing the post-aggregator, or nested aggregations that need comparison).

Common situations: Developers write an ORDER BY on the output column computed by the CDF post-aggregator, or build a query generator that automatically assigns comparators to all post-aggregators, unaware that array-valued post-aggregators (CDFs, histograms, quantiles) are not comparable.

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/cb5caf3f7bbb2043. Report an issue: GitHub.