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

Can't get sketch from object of type [%s]

Error message

Can't get sketch from object of type [%s]

What it means

SketchHolder.getSketch() converts the holder's underlying object into an Apache DataSketches Theta Sketch. It only knows how to handle Sketch, Union, and Memory instances; anything else reaches the final else branch and throws an ISE. This indicates the holder was constructed with (or a value deserialized into) an object of an unsupported type.

Source

Thrown at extensions-core/datasketches/src/main/java/org/apache/druid/query/aggregation/datasketches/theta/SketchHolder.java:155

    } else {
      union.union(getSketch());
    }
  }

  public Sketch getSketch()
  {
    if (cachedSketch != null) {
      return cachedSketch;
    }

    if (obj instanceof Sketch) {
      cachedSketch = (Sketch) obj;
    } else if (obj instanceof Union) {
      cachedSketch = ((Union) obj).getResult();
    } else if (obj instanceof Memory) {
      cachedSketch = deserializeFromMemory((Memory) obj);
    } else {
      throw new ISE("Can't get sketch from object of type [%s]", obj.getClass().getName());
    }
    return cachedSketch;
  }

  public double getEstimate()
  {
    if (cachedEstimate == null) {
      cachedEstimate = getSketch().getEstimate();
    }
    return cachedEstimate.doubleValue();
  }

  public SketchEstimateWithErrorBounds getEstimateWithErrorBounds(int errorBoundsStdDev)
  {
    Sketch sketch = getSketch();
    SketchEstimateWithErrorBounds result = new SketchEstimateWithErrorBounds(
        getEstimate(),
        sketch.getUpperBound(errorBoundsStdDev),

View on GitHub (pinned to 9b90983fd2)

Solutions

1. Check what object was passed to SketchHolder.of() and ensure it is a Sketch, Union, or Memory
2. Unwrap nested SketchHolder values with holder.getSketch() before passing them to of()
3. Align the datasketches-java dependency version across all Druid nodes so serialization round-trips produce known types
4. Log obj.getClass().getName() (it is already in the message) and add a type check before constructing the holder

Example fix

// before
SketchHolder holder = SketchHolder.of(obj); // obj may be anything
Sketch sketch = holder.getSketch(); // may throw ISE
// after
if (obj instanceof SketchHolder) {
  obj = ((SketchHolder) obj).getSketch();
}
if (!(obj instanceof Sketch) && !(obj instanceof Union) && !(obj instanceof Memory)) {
  throw new IllegalArgumentException("Unsupported sketch object: " + obj.getClass());
}
SketchHolder holder = SketchHolder.of(obj);

if (!(obj instanceof Sketch) && !(obj instanceof Union) && !(obj instanceof Memory)) {
  throw new IllegalArgumentException("Unsupported sketch object: " + obj.getClass().getName());
}

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

try {
  Sketch s = holder.getSketch();
} catch (IllegalStateException e) {
  // log obj class from message; fall back to re-wrapping via Memory.wrap(rawBytes)
}

When it happens

Trigger: Calling getSketch() (directly or via getEstimate(), equals(), hashCode(), or set operations) on a SketchHolder whose wrapped object is not a Sketch, Union, or Memory — e.g. after SketchHolder.of(someArbitraryObject) or deserializing a payload produced by an incompatible sketch version.

Common situations: Mixing DataSketches versions where serialized bytes yield an unexpected class; passing a nested SketchHolder instead of the raw sketch object; custom aggregation code stuffing a non-sketch object into SketchHolder.of(); classloader differences in federated/cluster deployments returning an unrecognized implementation.

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

• Illegal type received while theta sketch merging [%s]
• Unknwon types [%s] and [%s]
• Cannot deserialize type[%s] to an RoaringBitmap64Counter:
• Expected a number or an instance of MergingDigest, but recei
• Invalid input [%s] of type [%s] for [%s] aggregator [%s]

AI-assisted analysis of apache/druid@9b90983fd2 (2026-09-07). Data as JSON: /api/errors/75af7c31d37a4851. Report an issue: GitHub.