apache/druid · error · java.lang.UnsupportedOperationException

ApproximateHistogramFoldingAggregator does not support getFl

Error message

ApproximateHistogramFoldingAggregator does not support getFloat()

What it means

ApproximateHistogramFoldingAggregator folds ApproximateHistogram objects at query time; its result is a histogram, not a primitive float. getFloat() is intentionally unimplemented and throws UnsupportedOperationException whenever called. Numeric values must be derived via post-aggregation on the folded histogram.

Source

Thrown at extensions-core/histogram/src/main/java/org/apache/druid/query/aggregation/histogram/ApproximateHistogramFoldingAggregator.java:72

    }

    if (h.binCount() + histogram.binCount() <= tmpBufferB.length) {
      histogram.foldFast(h, tmpBufferP, tmpBufferB);
    } else {
      histogram.foldFast(h);
    }
  }

  @Override
  public Object get()
  {
    return histogram;
  }

  @Override
  public float getFloat()
  {
    throw new UnsupportedOperationException("ApproximateHistogramFoldingAggregator does not support getFloat()");
  }

  @Override
  public long getLong()
  {
    throw new UnsupportedOperationException("ApproximateHistogramFoldingAggregator does not support getLong()");
  }

  @Override
  public double getDouble()
  {
    throw new UnsupportedOperationException("ApproximateHistogramFoldingAggregator does not support getDouble()");
  }

  @Override
  public void close()
  {
    // no resources to cleanup

View on GitHub (pinned to 9b90983fd2)

Solutions

Use histogram post-aggregators (quantile, min, max, etc.) on the folded histogram to get numeric results.
Switch to a numeric aggregator (doubleSum, etc.) if a float/double output is actually needed.
Catch UnsupportedOperationException in generic aggregation loops and skip non-numeric aggregators.

Example fix

// before
float v = foldingAggregator.getFloat();
// after
// in query JSON use:
// {"type": "quantile", "fieldName": "histFold", "probability": 0.5}
float v = (float) quantileResult;

Defensive patterns

Strategy: validation

Validate before calling

// Guard before calling getFloat on a folding aggregator:
if (aggregator instanceof org.apache.druid.query.aggregation.histogram.ApproximateHistogramFoldingAggregator) {
  // not supported; use post-aggregation instead
}

Type guard

boolean supportsFloat(Object agg) {
  return !(agg instanceof org.apache.druid.query.aggregation.histogram.ApproximateHistogramFoldingAggregator);
}

Try / catch

try {
  value = aggregator.getFloat();
} catch (UnsupportedOperationException e) {
  value = (float) histogramQuantile(aggregator.get());
}

Prevention

Only query approxHistogramFold results through histogram post-aggregators.
When migrating queries from numeric aggregators, update all value-extraction paths, not just the aggregation spec.
Check AggregatorFactory.getType() before selecting an accessor.

When it happens

Trigger: Calling ApproximateHistogramFoldingAggregator.getFloat(), or a query path requesting a float accessor on an approxHistogramFold aggregation.

Common situations: Using approxHistogramFold where a float metric aggregator is expected; generic code that calls getFloat() on every Aggregator; older queries migrated from numeric aggregators to histogram aggregators without updating output handling.

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/1e6d8f12a146f678. Report an issue: GitHub.