apache/druid · error · java.lang.UnsupportedOperationException

Unknown type:

Error message

Unknown type: 

What it means

FixedBucketsHistogramSerde's extractValue throws UnsupportedOperationException('Unknown type: <class>') when the raw column value is neither a FixedBucketsHistogram, a String (base64 or JSON-encoded histogram), nor another recognized type. The serde only knows how to deserialize histograms from a small set of input representations.

Source

Thrown at extensions-core/histogram/src/main/java/org/apache/druid/query/aggregation/histogram/FixedBucketsHistogramSerde.java:117

        } else if (rawValue instanceof String) {
          Number numberAttempt;
          try {
            numberAttempt = Rows.objectToNumber(metricName, rawValue, true);
            FixedBucketsHistogram fbh = new FixedBucketsHistogram(
                aggregatorFactory.getLowerLimit(),
                aggregatorFactory.getUpperLimit(),
                aggregatorFactory.getNumBuckets(),
                aggregatorFactory.getOutlierHandlingMode()
            );
            fbh.add(numberAttempt.doubleValue());
            return fbh;
          }
          catch (ParseException pe) {
            FixedBucketsHistogram fbh = FixedBucketsHistogram.fromBase64((String) rawValue);
            return fbh;
          }
        } else {
          throw new UnsupportedOperationException("Unknown type: " + rawValue.getClass());
        }
      }
    };
  }

  @Override
  public ObjectStrategy getObjectStrategy()
  {
    return new ObjectStrategy<FixedBucketsHistogram>()
    {
      @Override
      public Class<? extends FixedBucketsHistogram> getClazz()
      {
        return FixedBucketsHistogram.class;
      }

      @Override
      public FixedBucketsHistogram fromByteBuffer(ByteBuffer buffer, int numBytes)

View on GitHub (pinned to 9b90983fd2)

Solutions

1. Ensure the raw value is a String containing a base64-encoded FixedBucketsHistogram or a JSON-encoded histogram (or an actual FixedBucketsHistogram object).
2. Fix the upstream producer/ingestion spec so the histogram is serialized with FixedBucketsHistogram.toBase64() before ingestion.
3. Add a preprocessing step (e.g. a parseSpec/flatten transform) that converts the value to the expected base64 string.

Example fix

// before: ingesting a raw number
row.add("myHist", 42);
// after
row.add("myHist", FixedBucketsHistogram.toBase64(fbh));

Object raw = row.getRaw("myHist");
if (!(raw == null || raw instanceof FixedBucketsHistogram || raw instanceof String)) {
  throw new IllegalArgumentException("myHist must be base64/JSON histogram string, got " + raw.getClass());
}

boolean isHistogramValue(Object v) {
  return v instanceof FixedBucketsHistogram ||
         (v instanceof String); // base64 or JSON-encoded histogram
}

try {
  Object v = serde.extractValue(row, "myHist", aggFactory);
} catch (UnsupportedOperationException e) {
  // handle/log unexpected raw type, e.g. re-serialize producer data
}

When it happens

Trigger: Ingesting rows where the metric column holds a value of an unexpected Java class (e.g. Number, Map, byte[]) for a fixedBucketsHistogram metric; the serde falls through all instanceof checks and throws.

Common situations: Ingestion specs with wrong-type input (numeric field mapped to a histogram metric), programmatic row construction putting raw objects in, or upstream producers writing the histogram in an unencodable format.

Understand the failure class

Background: Type mismatch errors: IllegalArgumentException, TypeError and type guards across 150 open-source libraries — this error's family across 150 libraries.

Related errors

• extractValue without an aggregator factory is not supported.
• Unknown value type:
• Unknown value type:
• Cannot deserialize type[%s] to an RoaringBitmap64Counter:
• Expected a number or an instance of MergingDigest, but recei

AI-assisted analysis of apache/druid@9b90983fd2 (2026-09-07). Data as JSON: /api/errors/43e3b6823deda459. Report an issue: GitHub.