apache/druid · error · java.lang.UnsupportedOperationException

CardinalityBufferAggregators does not support getDouble()

Error message

CardinalityBufferAggregators does not support getDouble()

What it means

CardinalityBufferAggregator's getDouble() is unsupported: the buffer holds an HLL sketch, not a double, so the method always throws UnsupportedOperationException. (The message contains a pre-existing typo: 'CardinalityBufferAggregators'.)

Source

Thrown at processing/src/main/java/org/apache/druid/query/aggregation/cardinality/CardinalityBufferAggregator.java:98

  }

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


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

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

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

  @Override
  public void inspectRuntimeShape(RuntimeShapeInspector inspector)
  {
    for (int i = 0; i < selectorPluses.length; i++) {
      inspector.visit(StringUtils.format("selector-%d", i), selectorPluses[i].getSelector());
    }
  }
}

View on GitHub (pinned to 9b90983fd2)

Solutions

Read the sketch via get(buf, position) instead of getDouble()
Use a numeric aggregator if a double result is required
Finalize the sketch into an estimate before numeric consumption

Example fix

// before
double v = bufferAggregator.getDouble(buf, position);
// after
Object v = bufferAggregator.get(buf, position); // HLL sketch

Defensive patterns

Strategy: type-guard

Validate before calling

if (!(aggFactory instanceof CardinalityAggregatorFactory)) { double v = bufferAggregator.getDouble(buf, position); }

Type guard

if (factory instanceof CardinalityAggregatorFactory) {
    Object v = bufferAggregator.get(buf, position);
} else {
    double v = bufferAggregator.getDouble(buf, position);
}

Try / catch

try {
    v = bufferAggregator.getDouble(buf, position);
} catch (UnsupportedOperationException e) {
    v = Double.NaN;
}

Prevention

Use double aggregators for double-typed columns
Never call getDouble on sketch-based buffer aggregators
Keep result extraction generic via get() when aggregation type is unknown

When it happens

Trigger: Calling bufferAggregator.getDouble(buf, position) on a cardinality buffer aggregator when a double-typed result is assumed.

Common situations: Vectorized double extraction on cardinality aggregations; generic aggregation pipelines that request double values from all aggregators.

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/d027d538c4cb42ed. Report an issue: GitHub.