apache/druid · error · org.apache.druid.java.util.common.ISE

Object is not of a type

Error message

Object is not of a type[%s] that can be deserialized to sketch.

What it means

SketchHolder.deserialize() accepts an object that should be a serialized sketch form (Sketch, Union, or Memory) and wraps it in a SketchHolder. If the object is none of these types, it throws an ISE because there is no defined way to turn it into a theta sketch. It is the entry-point validation used by deserializeSafe().

Solutions

Ensure the input is a Sketch, Union, or Memory; if you have raw bytes, wrap them with Memory.wrap(byte[]) before deserializing
If the value is a JSON string, decode it into Memory/sketch bytes rather than passing the String directly
Verify the input source (segment/store) was written by the same DataSketches family and version
Add an instanceof check on the value and route unsupported types to a fallback or error path before calling deserialize

Example fix

// before
SketchHolder holder = SketchHolder.deserialize(rawObject); // rawObject is byte[]
// after
Object obj = rawObject instanceof byte[]
    ? Memory.wrap((byte[]) rawObject)
    : rawObject;
SketchHolder holder = SketchHolder.deserialize(obj);

Defensive patterns

Strategy: type-guard

Validate before calling

if (!(serializedSketch instanceof Sketch)
    && !(serializedSketch instanceof Union)
    && !(serializedSketch instanceof Memory)) {
  throw new IllegalArgumentException("Not deserializable to theta sketch: " + serializedSketch.getClass());
}

Type guard

boolean isDeserializableToSketch(Object o) {
  return o instanceof Sketch || o instanceof Union || o instanceof Memory;
}

Try / catch

try {
  SketchHolder h = SketchHolder.deserializeSafe(obj);
} catch (IllegalStateException e) {
  // inspect obj.getClass(); convert raw bytes via Memory.wrap() and retry once
}

Prevention

Wrap raw byte[] in Memory.wrap() before deserialization
Never pass JSON Strings/Maps directly to deserialize()
Keep sketch serialization formats consistent between writer and reader systems
Use deserializeSafe() which handles null and returns an empty holder for null inputs

When it happens

Trigger: Calling SketchHolder.deserialize(obj) or deserializeSafe(obj) with anything other than a Sketch, Union, or Memory instance — typically a String, byte[], Map, or ByteBuffer coming from JSON ingestion of an aggregator column or a custom deserialization path.

Common situations: Ingesting theta sketch columns serialized by a different system where the value deserialized into a generic JSON type (String/List) instead of Memory; feeding raw byte[] (Druid deserializes Memory itself, so a raw array reaches this as byte[]); storing sketches in a serialization format the extension cannot recognize.

Understand the failure class

Background: "is not a compatible type" / "cannot merge" errors: when a value's type doesn't match what the library requires — this error's family across 65 libraries.

Related errors

AI-assisted analysis of apache/druid@9b90983fd2 (2026-09-07). Data as JSON: /api/errors/a8cdad2437c7abda. Report an issue: GitHub.

Appendix: source

Thrown at extensions-core/datasketches/src/main/java/org/apache/druid/query/aggregation/datasketches/theta/SketchHolder.java:222

    cachedEstimate = null;
    cachedSketch = null;
  }

  public static SketchHolder deserialize(Object serializedSketch)
  {
    if (serializedSketch instanceof String) {
      return SketchHolder.of(deserializeFromBase64EncodedString((String) serializedSketch));
    } else if (serializedSketch instanceof byte[]) {
      return SketchHolder.of(deserializeFromByteArray((byte[]) serializedSketch));
    } else if (serializedSketch instanceof SketchHolder) {
      return (SketchHolder) serializedSketch;
    } else if (serializedSketch instanceof Sketch
               || serializedSketch instanceof Union
               || serializedSketch instanceof Memory) {
      return SketchHolder.of(serializedSketch);
    }

    throw new ISE(
        "Object is not of a type[%s] that can be deserialized to sketch.",
        serializedSketch.getClass()
    );
  }

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

    return deserialize(serializedSketch);
  }

  private static Sketch deserializeFromBase64EncodedString(String str)
  {

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