apache/druid · error · IllegalStateException

Object is not of a type that can be deserialized to a quanti

Error message

Object is not of a type that can be deserialized to a quantiles DoublesSketch: %s

What it means

KllDoublesSketchOperations.deserialize only accepts a base64-encoded String, a byte[], or an already-built KllDoublesSketch. Any other object type (including null) cannot be converted to a sketch, so it throws IllegalStateException. This is a defensive check in the post-aggregator/initializer path where serialized sketch state is coerced.

Source

Thrown at extensions-core/datasketches/src/main/java/org/apache/druid/query/aggregation/datasketches/kll/KllDoublesSketchOperations.java:44

import org.apache.druid.segment.data.SafeWritableMemory;

import java.nio.charset.StandardCharsets;

public class KllDoublesSketchOperations
{

  public static final KllDoublesSketch EMPTY_SKETCH = KllDoublesSketch.newHeapInstance();

  public static KllDoublesSketch 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 KllDoublesSketch) {
      return (KllDoublesSketch) serializedSketch;
    }
    throw new ISE(
        "Object is not of a type that can be deserialized to a quantiles DoublesSketch: %s",
        serializedSketch == null ? "null" : serializedSketch.getClass()
    );
  }

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

View on GitHub (pinned to 9b90983fd2)

Solutions

1. Verify the underlying column is ingested with the kllDoublesSketch aggregator so stored values are sketch payloads
2. Pass base64 String, byte[], or KllDoublesSketch explicitly; null-check before calling deserialize
3. Use deserializeSafe if you want null/empty handling instead of raw deserialize
4. Confirm you are not mixing the datasketches-quantiles DoublesSketch with KllDoublesSketch types

Example fix

// before
KllDoublesSketch sketch = KllDoublesSketchOperations.deserialize(someMap.get("col"));
// after
Object val = someMap.get("col");
KllDoublesSketch sketch = (val instanceof String || val instanceof byte[] || val instanceof KllDoublesSketch)
    ? KllDoublesSketchOperations.deserialize(val) : null;

Object v = input;
if (!(v instanceof String || v instanceof byte[] || v instanceof KllDoublesSketch)) {
  throw new IllegalArgumentException("Unsupported sketch input: " + (v == null ? "null" : v.getClass()));
}

boolean isDeserializable(Object o) { return o instanceof String || o instanceof byte[] || o instanceof KllDoublesSketch; }

try { sketch = KllDoublesSketchOperations.deserialize(obj); } catch (IllegalStateException e) { log.warn("Bad sketch input", e); sketch = null; }

When it happens

Trigger: Passing an Object that is none of String/byte[]/KllDoublesSketch to KllDoublesSketchOperations.deserialize — e.g. a List, Map, Number, or null, usually because the wrong field was fed to a sketch post-aggregator or a column was ingested as a non-sketch type.

Common situations: Pointing a kllDoublesSketch post-aggregator at a plain numeric/string column instead of a sketch-typed column; query results where the intermediate field was serialized to JSON as a non-sketch structure; accidentally passing a QuantilesSketch (different extension) object.

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

• 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
• Object is not of a type that can deserialize to sketch: %s
• 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/cea4975e95c105e8. Report an issue: GitHub.