apache/druid · error · java.lang.UnsupportedOperationException

ApproximateHistogramFoldingBufferAggregator does not…

Error message

ApproximateHistogramFoldingBufferAggregator does not support getFloat()

What it means

ApproximateHistogramFoldingBufferAggregator folds histograms into a ByteBuffer; getFloat() is intentionally unsupported because the buffered result is a histogram object, not a float. Calling it always throws UnsupportedOperationException. Use post-aggregation to extract numeric statistics from the folded histogram.

Solutions

  1. Use histogram post-aggregators (quantile, min, max, etc.) to compute numeric values from the folded histogram.
  2. Aggregate the numeric column directly with a float/double-capable aggregator if the histogram is unnecessary.
  3. Wrap generic accessor calls in a try/catch for UnsupportedOperationException and use object-based access instead.

Example fix

// before
float v = bufferAggregator.getFloat(buf, position);
// after
// query JSON:
// {"type": "quantile", "fieldName": "histFold", "probability": 0.9}
float v = (float) p90Result;
Defensive patterns

Strategy: validation

Validate before calling

// Guard buffered fold aggregators before primitive getters:
if (aggregator instanceof org.apache.druid.query.aggregation.histogram.ApproximateHistogramFoldingBufferAggregator) {
  // use histogram post-aggregators instead of getFloat()
}

Type guard

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

Try / catch

try {
  value = aggregator.getFloat(buf, position);
} catch (UnsupportedOperationException e) {
  value = (float) quantileFromBuffer(buf, position, 0.5);
}

Prevention

When it happens

Trigger: Calling ApproximateHistogramFoldingBufferAggregator.getFloat(ByteBuffer, int), or a query engine requesting a float accessor from a buffer-aggregated approxHistogramFold column.

Common situations: Generic code iterating all buffer aggregators numerically; queries that treat histogram fold results as float metrics; frameworks auto-selecting float accessors based on column naming conventions.

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/95b447ca62d31af0. Report an issue: GitHub.

Appendix: source

Thrown at extensions-core/histogram/src/main/java/org/apache/druid/query/aggregation/histogram/ApproximateHistogramFoldingBufferAggregator.java:66

  }

  @Override
  public void aggregate(ByteBuffer buf, int position)
  {
    ApproximateHistogram hNext = selector.getObject();
    innerAggregator.aggregate(buf, position, hNext);
  }

  @Override
  public Object get(ByteBuffer buf, int position)
  {
    return innerAggregator.get(buf, position);
  }

  @Override
  public float getFloat(ByteBuffer buf, int position)
  {
    throw new UnsupportedOperationException("ApproximateHistogramFoldingBufferAggregator does not support getFloat()");
  }

  @Override
  public long getLong(ByteBuffer buf, int position)
  {
    throw new UnsupportedOperationException("ApproximateHistogramFoldingBufferAggregator does not support getLong()");
  }

  @Override
  public double getDouble(ByteBuffer buf, int position)
  {
    throw new UnsupportedOperationException("ApproximateHistogramFoldingBufferAggregator does not support getDouble()");
  }

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

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