apache/druid · error · IAE

Comparing arrays of estimates and error bounds is not suppor

Error message

Comparing arrays of estimates and error bounds is not supported

What it means

HllSketchToEstimateWithBoundsPostAggregator outputs double[] (estimate plus upper/lower bounds), and since arrays of estimates+bounds have no meaningful ordering, getComparator() always throws IllegalArgumentException. This post-aggregator cannot be used where comparability is required, e.g. as the field of an orderBy/sorting or comparator-consuming aggregator.

Source

Thrown at extensions-core/datasketches/src/main/java/org/apache/druid/query/aggregation/datasketches/hll/HllSketchToEstimateWithBoundsPostAggregator.java:100

    return field;
  }

  @JsonProperty
  public int getNumStdDev()
  {
    return numStdDevs;
  }

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

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

  @Override
  public double[] compute(final Map<String, Object> combinedAggregators)
  {
    Object hllSketchHolderObject = field.compute(combinedAggregators);
    if (hllSketchHolderObject == null) {
      return new double[] {0.0D, 0.0D, 0.0D};
    }
    final HllSketchHolder sketch = HllSketchHolder.fromObj(hllSketchHolderObject);
    return new double[] {sketch.getEstimate(), sketch.getLowerBound(numStdDevs), sketch.getUpperBound(numStdDevs)};
  }

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

View on GitHub (pinned to 9b90983fd2)

Solutions

  1. Use HllSketchToEstimatePostAggregator (double estimate) where sorting/comparison is needed
  2. Sort by a scalar projection (the estimate) rather than the double[] bounds array
  3. If bounds are needed, output them as separate columns and sort by the estimate column

Example fix

// before
postAggs.add(new HllSketchToEstimateWithBoundsPostAggregator("est", field, 2));
query.orderBy("est")
// after
postAggs.add(new HllSketchToEstimatePostAggregator("est", field));
query.orderBy("est")
Defensive patterns

Strategy: validation

Validate before calling

if (postAgg instanceof HllSketchToEstimateWithBoundsPostAggregator) { throw new IllegalArgumentException("estimate-with-bounds output is not comparable; use HllSketchToEstimatePostAggregator for sorting"); }

Try / catch

try { cmp = postAgg.getComparator(); } catch (IllegalArgumentException e) { cmp = Comparator.comparingDouble(a -> ((double[]) a)[0]); } // compare by estimate only

Prevention

When it happens

Trigger: Using HllSketchToEstimateWithBounds (estimate+bounds output) inside an ORDER BY / having spec, or feeding it into another post-aggregator that calls getComparator() (e.g. arithmetic or comparative post-aggs).

Common situations: Users trying to sort query results by estimate-with-bounds; wrapping the bounds post-agg in a greatest/least or comparison post-aggregator; streaming frameworks requesting a comparator for the post-aggregated column.

Related errors

AI-assisted analysis of apache/druid@9b90983fd2 (2026-09-07). Data as JSON: /api/errors/13dfb37a3faff382. Report an issue: GitHub.