apache/druid · warning · UnsupportedOperationException

Not implemented

Error message

Not implemented

What it means

MomentSketchMergeAggregator.getFloat throws UnsupportedOperationException because merged Moments Sketch results are composite objects that cannot be exposed as a float. The merge aggregator combines sketches; any attempt to read its value as a float triggers this error.

Source

Thrown at extensions-contrib/momentsketch/src/main/java/org/apache/druid/query/aggregation/momentsketch/aggregator/MomentSketchMergeAggregator.java:61

  public void aggregate()
  {
    final MomentSketchWrapper sketch = selector.getObject();
    if (sketch == null) {
      return;
    }
    this.momentsSketch.merge(sketch);
  }

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

  @Override
  public float getFloat()
  {
    throw new UnsupportedOperationException("Not implemented");
  }

  @Override
  public long getLong()
  {
    throw new UnsupportedOperationException("Not implemented");
  }

  @Override
  public void close()
  {
    momentsSketch = null;
  }
}

View on GitHub (pinned to 9b90983fd2)

Solutions

  1. Keep the sketch aggregation output as COMPLEX and read merged sketches via getObject.
  2. Replace float coercion with sketch-specific post-aggregators (quantile, mean).
  3. Remove any expression or cast that treats the merged sketch column as a float.
  4. Use MomentSketchAggregatorFactory's merge/finalize APIs rather than numeric accessors.

Example fix

// before
PostAggregator p = (Float) row.get("sketch"); // engine calls getFloat
// after
MomentSketchWrapper sketch = (MomentSketchWrapper) row.get("sketch");
double mean = sketch.mean();
Defensive patterns

Strategy: type-guard

Validate before calling

if ("FLOAT".equals(querySpec.getOutputTypeOf("sketch"))) {
  throw new IllegalArgumentException("merged momentsketch requires COMPLEX output");
}

Type guard

Object val = row.get("sketch");
if (val instanceof MomentSketchWrapper) {
  MomentSketchWrapper sketch = (MomentSketchWrapper) val; // safe, numeric via post-aggregator
}

Try / catch

try {
  return mergeAggregator.getFloat();
} catch (UnsupportedOperationException e) {
  throw new IllegalStateException("Read merged sketch via getObject, not getFloat", e);
}

Prevention

When it happens

Trigger: A query performs sketch merging (e.g. group-by across segments or a final merge stage) and requests the merged metric as FLOAT, causing getFloat() on the merge aggregator.

Common situations: Group-by queries with sketch aggregations coerced to float output; finalization/post-aggregation code calling getFloat on merged rows; query tooling assuming numeric aggregator types.

Related errors

AI-assisted analysis of apache/druid@9b90983fd2 (2026-09-07). Data as JSON: /api/errors/14a289e69b2fe131. Report an issue: GitHub.