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

Comparing sketch summaries is not supported

Error message

Comparing sketch summaries is not supported

What it means

DoublesSketchToStringPostAggregator outputs a human-readable sketch summary String. Strings could be compared alphabetically, but the summary is diagnostic text whose ordering is meaningless, so getComparator() deliberately throws IllegalArgumentException('Comparing sketch summaries is not supported').

Solutions

1. Sort by a scalar post-aggregator (quantile/histogram-derived value) from the same sketch.
2. Remove the ORDER BY / metric reference to the string post-aggregator.
3. If you just need the summary for display, keep it in postAggregators but exclude it from sorting specs.

Example fix

// before
{"queryType":"topN","metric":"sketchSummary", ...}
// after
{"queryType":"topN","metric":"quantile50", "postAggregators":[{"type":"quantilesDoublesSketchToString",...}]}

if (sortKeys.contains(toStringPostAggName)) { throw new IllegalArgumentException("Sketch summaries are not sortable"); }

try { druidClient.run(query); } catch (IllegalArgumentException e) { if (e.getMessage().contains("Comparing sketch summaries")) { /* drop sort key and rerun */ } }

When it happens

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

Common situations: ORDER BY on the sketch summary column in SQL or native queries; treating the toString output as a sortable dimension.

Related errors

• Comparing arrays of quantiles is not supported
• Comparing histograms is not supported
• Comparing histograms 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/54bbbcdcf9658bf1. Report an issue: GitHub.

Appendix: source

Thrown at extensions-core/datasketches/src/main/java/org/apache/druid/query/aggregation/datasketches/quantiles/DoublesSketchToStringPostAggregator.java:83

  }

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

  @Override
  public Object compute(final Map<String, Object> combinedAggregators)
  {
    final DoublesSketch sketch = (DoublesSketch) field.compute(combinedAggregators);
    return sketch.toString();
  }

  @Override
  public Comparator<String> getComparator()
  {
    throw new IAE("Comparing sketch summaries is not supported");
  }

  @Override
  public byte[] getCacheKey()
  {
    final CacheKeyBuilder builder = new CacheKeyBuilder(
        AggregatorUtil.QUANTILES_DOUBLES_SKETCH_TO_STRING_CACHE_TYPE_ID).appendCacheable(field);
    return builder.build();
  }

  @Override
  public PostAggregator decorate(final Map<String, AggregatorFactory> map)
  {
    return this;
  }

  @Override
  public Set<String> getDependentFields()

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