apache/druid · error · IllegalArgumentException

Object cannot be deserialized to a TDigest Sketch:

Error message

Object cannot be deserialized to a TDigest Sketch: 

What it means

TDigestSketchUtils.deserialize() is a generic type-guard/normalizer: it accepts a base64 String, a raw byte[], or an already-decoded MergingDigest. The error fires for any other runtime type (or a payload that fails the accepted decoding path), meaning the stored or transmitted sketch value is not in a format this extension can turn into a MergingDigest.

Solutions

1. Ensure persisted sketch values are base64 strings or byte[] produced by the tdigestsketch serde
2. If the value is a corrupted or truncated base64/byte payload, re-aggregate or re-ingest the affected rows
3. Check that no intermediary step (e.g. custom code or a different serde) is substituting another object type for the sketch

When it happens

Trigger: Thrown at extensions-contrib/tdigestsketch/src/main/java/org/apache/druid/query/aggregation/tdigestsketch/TDigestSketchUtils.java:47 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of apache/druid@9b90983fd2 (2026-09-07). Data as JSON: /api/errors/f14ff4aefc1b6e06. Report an issue: GitHub.

Appendix: source

Thrown at extensions-contrib/tdigestsketch/src/main/java/org/apache/druid/query/aggregation/tdigestsketch/TDigestSketchUtils.java:47

public class TDigestSketchUtils
{
  // Class is not meant to be instantiated
  private TDigestSketchUtils()
  {
  }

  public static MergingDigest deserialize(Object serializedSketch)
  {
    if (serializedSketch instanceof String) {
      String str = (String) serializedSketch;
      return MergingDigest.fromBytes(ByteBuffer.wrap(StringUtils.decodeBase64(StringUtils.toUtf8(str))));
    } else if (serializedSketch instanceof byte[]) {
      return MergingDigest.fromBytes(ByteBuffer.wrap((byte[]) serializedSketch));
    } else if (serializedSketch instanceof MergingDigest) {
      return (MergingDigest) serializedSketch;
    }
    throw new IAE(
        "Object cannot be deserialized to a TDigest Sketch: "
        + serializedSketch.getClass()
    );
  }

  static byte[] toBytes(MergingDigest tDigest)
  {
    byte[] arr = new byte[tDigest.byteSize()];
    ByteBuffer result = ByteBuffer.wrap(arr);
    tDigest.asBytes(result);
    return result.array();
  }

  /**
   * This method computes an estimate of the max intermediate size of a {@link MergingDigest}.
   * Since there is no utility available in the T-Digest library to compute this size,
   * the below code is inspired by looking at
   * {@link MergingDigest#MergingDigest(double, int, int)}

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