apache/druid · error · IllegalStateException

Object is not of a type that can deserialize to sketch: %s

Error message

Object is not of a type that can deserialize to sketch: %s

What it means

ArrayOfDoublesSketchOperations.deserialize() only accepts String (base64), byte[], or an already-deserialized ArrayOfDoublesSketch. Any other object type reaches the final ISE, which reports the offending class. This is a type-guard failure in sketch deserialization.

Source

Thrown at extensions-core/datasketches/src/main/java/org/apache/druid/query/aggregation/datasketches/tuple/ArrayOfDoublesSketchOperations.java:116

    {
      final double[] result = new double[a.length];
      for (int i = 0; i < a.length; i++) {
        result[i] = a[i] + b[i];
      }
      return result;
    }
  };

  public static ArrayOfDoublesSketch deserialize(final Object serializedSketch)
  {
    if (serializedSketch instanceof String) {
      return deserializeFromBase64EncodedString((String) serializedSketch);
    } else if (serializedSketch instanceof byte[]) {
      return deserializeFromByteArray((byte[]) serializedSketch);
    } else if (serializedSketch instanceof ArrayOfDoublesSketch) {
      return (ArrayOfDoublesSketch) serializedSketch;
    }
    throw new ISE("Object is not of a type that can deserialize to sketch: %s", serializedSketch.getClass());
  }

  public static ArrayOfDoublesSketch deserializeSafe(final Object serializedSketch)
  {
    if (serializedSketch instanceof String) {
      return deserializeFromBase64EncodedStringSafe((String) serializedSketch);
    } else if (serializedSketch instanceof byte[]) {
      return deserializeFromByteArraySafe((byte[]) serializedSketch);
    }

    return deserialize(serializedSketch);
  }

  public static ArrayOfDoublesSketch deserializeFromBase64EncodedString(final String str)
  {
    return deserializeFromByteArray(StringUtils.decodeBase64(str.getBytes(StandardCharsets.UTF_8)));
  }

View on GitHub (pinned to 9b90983fd2)

Solutions

1. Inspect the reported class name in the message to see what was actually passed
2. Use deserializeSafe() which returns null instead of throwing for unusable objects
3. Ensure the source column/aggregator genuinely produces ArrayOfDoublesSketch objects
4. Convert the object to byte[] or base64 String before deserializing

Example fix

// before
ArrayOfDoublesSketch sketch = ArrayOfDoublesSketchOperations.deserialize(obj);
// after
ArrayOfDoublesSketch sketch = ArrayOfDoublesSketchOperations.deserializeSafe(obj);
if (sketch == null) {
  // handle non-sketch object
}

if (!(obj instanceof String) && !(obj instanceof byte[]) && !(obj instanceof ArrayOfDoublesSketch)) {
  throw new IllegalStateException("Cannot deserialize to sketch: " + obj.getClass());
}

boolean isDeserializable = obj instanceof String
    || obj instanceof byte[]
    || obj instanceof ArrayOfDoublesSketch;

try {
  ArrayOfDoublesSketch s = ArrayOfDoublesSketchOperations.deserialize(obj);
} catch (IllegalStateException e) {
  // handle non-sketch object, e.g. log and treat as empty/null
}

When it happens

Trigger: Calling deserialize(Object) with an object that is none of String, byte[], or ArrayOfDoublesSketch — e.g. a Map, List, or Numeric value returned by an expression or stored in a column.

Common situations: Query results where the column does not actually contain a serialized sketch (wrong column chosen, aggregator changed, older data written with a different type); passing a Calcite/Java object of the wrong type into deserialize directly.

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

• Object is not of a type that can be deserialized to a quanti
• Object is not of a type that can be deserialized to a KllFlo
• Object is not of a type that can be deserialized to a quanti
• Cannot deserialize type[%s] to an RoaringBitmap64Counter:
• Object is not of a type[%s] that can be deserialized to sket

AI-assisted analysis of apache/druid@9b90983fd2 (2026-09-07). Data as JSON: /api/errors/d96dbc0c2ee01c93. Report an issue: GitHub.