apache/druid · error · ISE

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

Error message

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

What it means

fromObj only understands HllSketchHolder, HllSketch, Memory (off-heap sketch), and base64-encoded String inputs. Any other object type cannot be converted into a sketch, so an IllegalStateException with the offending class is thrown. This guards against silently misinterpreting values like Map or ByteBuffer as sketch payloads.

Source

Thrown at extensions-core/datasketches/src/main/java/org/apache/druid/query/aggregation/datasketches/hll/HllSketchHolder.java:52

    if (obj == null) {
      throw new NullPointerException("HllSketchHolder.fromObj cannot take a null argument");
    }

    if (obj instanceof HllSketchHolder) {
      return (HllSketchHolder) obj;
    } else if (obj instanceof HllSketch) {
      return HllSketchHolder.of((HllSketch) obj);
    } else if (obj instanceof Union) {
      return HllSketchHolder.of((Union) obj);
    } else if (obj instanceof byte[]) {
      return HllSketchHolder.of(HllSketch.heapify((byte[]) obj));
    } else if (obj instanceof Memory) {
      return HllSketchHolder.of(HllSketch.wrap((Memory) obj));
    } else if (obj instanceof String) {
      return HllSketchHolder.of(HllSketch.heapify(StringUtils.decodeBase64(StringUtils.toUtf8((String) obj))));
    }

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

  public static HllSketchHolder of(Union union)
  {
    return new HllSketchHolder(union, null);
  }

  public static HllSketchHolder of(HllSketch sketch)
  {
    return new HllSketchHolder(null, sketch);
  }

  private Union union;
  private HllSketch sketch;

  public HllSketchHolder(
      Union union,
      HllSketch sketch

View on GitHub (pinned to 9b90983fd2)

Solutions

Log/inspect obj.getClass() to identify the actual stored type and confirm the column contains HLL sketches
Ensure both writer and reader use the org.apache.druid.query.aggregation.datasketches.hll extension (not the legacy yahoo HLL extension)
If input is byte[] or ByteBuffer, wrap it in Memory (Memory.wrap) or base64-encode it into a String before calling fromObj

Example fix

// before
HllSketchHolder holder = HllSketchHolder.fromObj(byteArrayValue);
// after
HllSketchHolder holder = HllSketchHolder.fromObj(Memory.wrap(byteArrayValue));

Defensive patterns

Strategy: type-guard

Validate before calling

if (!(obj instanceof HllSketchHolder || obj instanceof HllSketch || obj instanceof Memory || obj instanceof String)) { throw new IllegalArgumentException("Unsupported sketch input: " + obj.getClass()); }

Type guard

boolean isDeserializableSketch(Object o) { return o instanceof HllSketchHolder || o instanceof HllSketch || o instanceof Memory || o instanceof String; }

Try / catch

try { holder = HllSketchHolder.fromObj(obj); } catch (IllegalStateException e) { log.warn("Bad sketch type: {}", e.getMessage()); throw e; }

Prevention

Verify the column was written by the same datasketches-hll extension version
Never pass byte[]/ByteBuffer directly; wrap with Memory.wrap first
Confirm the column name points at the sketch metric, not another field

When it happens

Trigger: Passing an object of an unsupported class (e.g. byte[], ByteBuffer, Map, StructuredData) to HllSketchHolder.fromObj, typically when the serialized column was produced by a different sketch type or an older/other extension (e.g. HyperLogLog vs HLL++).

Common situations: Mixing druid-datasketches versions where serialization format changed; ingesting metrics written by the old com.yahoo.sketches HLL extension; passing the wrong column name so a non-sketch field is fed to the aggregator.

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/4b1021c22167952f. Report an issue: GitHub.