apache/druid · error · java.lang.UnsupportedOperationException

StringFirstAggregator does not support getDouble()

Error message

StringFirstAggregator does not support getDouble()

What it means

StringFirstBufferAggregator stores a serialized string pair per group, so getDouble(ByteBuffer,int) is unsupported and always throws UnsupportedOperationException. Reading the buffer cell as a double is invalid by design.

Source

Thrown at processing/src/main/java/org/apache/druid/query/aggregation/firstlast/first/StringFirstBufferAggregator.java:127

    return StringFirstLastUtils.readPair(buf, position);
  }

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

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

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

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

  @Override
  public void inspectRuntimeShape(RuntimeShapeInspector inspector)
  {
    inspector.visit("timeSelector", timeSelector);
    inspector.visit("valueSelector", valueSelector);
  }
}

View on GitHub (pinned to 9b90983fd2)

Solutions

Read the cell via get(buf, position) instead of getDouble.
Remove DOUBLE outputType or arithmetic post-aggregations on stringFirst.
Use doubleFirst aggregation for numeric earliest values.
Check the aggregator's type before invoking numeric getters.

Example fix

// before
double v = bufAggregator.getDouble(buf, pos);
// after
Object v = bufAggregator.get(buf, pos); // SerializablePairLongString

Defensive patterns

Strategy: type-guard

Validate before calling

if (bufAggregator instanceof StringFirstBufferAggregator) {
  Object v = bufAggregator.get(buf, position);
} else {
  double v = bufAggregator.getDouble(buf, position);
}

Type guard

boolean supportsDouble(BufferAggregator a) {
  return !(a instanceof StringFirstBufferAggregator);
}

Try / catch

try {
  return agg.getDouble(buf, pos);
} catch (UnsupportedOperationException e) {
  Object pair = agg.get(buf, pos);
  return pair == null ? 0d : Double.parseDouble(String.valueOf(((SerializablePairLongString) pair).rhs));
}

Prevention

Do not declare DOUBLE outputType for stringFirst.
Use doubleFirst for double results.
Extract buffered string cells with get(buf, position).

When it happens

Trigger: Calling getDouble(buf, position) on a StringFirstBufferAggregator — usually a uniform numeric read path or DOUBLE outputType applied to stringFirst in a buffer-aggregated query.

Common situations: Group-by v2 with numeric outputType over stringFirst; shared result-extraction code calling getDouble on every aggregator cell.

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/39df7bf597f81569. Report an issue: GitHub.