What causes "Comparing histograms is not supported" in apache/druid?

Remove the ORDER BY / topN metric reference on the CDF post-aggregator; sort by the underlying doublesSketch aggregation's scalar post-agg or another scalar column instead.

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

DoublesSketchToCDFPostAggregator produces a cumulative distribution histogram (double[]). Druid's PostAggregator contract requires getComparator() for sorting/comparing results, but comparing two CDF arrays has no meaningful ordering, so the implementation always throws IllegalArgumentException('Comparing histograms is not supported').

Solutions

Remove the ORDER BY / topN metric reference on the CDF post-aggregator; sort by the underlying doublesSketch aggregation's scalar post-agg or another scalar column instead.
Sort by an accompanying scalar post-aggregator (e.g. quantile) derived from the same sketch.
Do client-side sorting after retrieving rows.

Example fix

// before
{"queryType":"topN","metric":"myCdf", ...} // throws
// after
{"queryType":"topN","metric":"myQuantile", // scalar post-agg on same sketch
 "postAggregators":[{"type":"quantilesDoublesSketchToQuantiles", ...}]}

Defensive patterns

Strategy: validation

Validate before calling

if (query.getOrderBy() != null && query.getOrderBy().contains("cdfPostAgg")) { throw new IllegalArgumentException("Cannot order by DoublesSketchToCDF output"); }

Try / catch

try { druidClient.run(query); } catch (IllegalArgumentException e) { if (e.getMessage().contains("Comparing histograms")) { /* strip ORDER BY on CDF and rerun */ } }

Prevention

Never reference array-valued post-aggregators in ORDER BY/topN metrics.
Order by scalar quantile post-aggs derived from the same sketch.

When it happens

Trigger: Using DoublesSketchToCDFPostAggregator in any context where the engine requests its comparator: ORDER BY on the post-aggregator output, topN metric on it, or sorting via having/limit specs that compare the column.

Common situations: Trying to ORDER BY a CDF/histogram column in SQL or native topN, or using the histogram as a sorting metric in a groupBy limit spec.

Related errors

AI-assisted analysis of apache/druid@9b90983fd2 (2026-09-07). Data as JSON: /api/errors/b08c60de13a38335. Report an issue: GitHub.

Appendix: source

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

    return ColumnType.DOUBLE_ARRAY;
  }

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

  @JsonProperty
  public double[] 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) +
        "}";
  }

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