apache/druid · error · IllegalArgumentException

Expected a number or an instance of MergingDigest, but recei

Error message

Expected a number or an instance of MergingDigest, but received [%s] of type [%s]

What it means

TDigestSketchAggregator's aggregate() accepts only numeric values (added as double data points) or MergingDigest instances (merged whole sketches). Any other object type in the selector is rejected with this IAE naming the value and its class.

Source

Thrown at extensions-contrib/tdigestsketch/src/main/java/org/apache/druid/query/aggregation/tdigestsketch/TDigestSketchAggregator.java:73

  }

  @Override
  public void aggregate()
  {
    Object obj = selector.getObject();
    if (obj == null) {
      return;
    }
    if (obj instanceof Number) {
      synchronized (this) {
        histogram.add(((Number) obj).doubleValue());
      }
    } else if (obj instanceof MergingDigest) {
      synchronized (this) {
        histogram.add((MergingDigest) obj);
      }
    } else {
      throw new IAE(
          "Expected a number or an instance of MergingDigest, but received [%s] of type [%s]",
          obj,
          obj.getClass()
      );
    }
  }

  @Nullable
  @Override
  public synchronized Object get()
  {
    return histogram;
  }

  @Override
  public float getFloat()
  {
    throw new UnsupportedOperationException("Casting to float type is not supported");

View on GitHub (pinned to 9b90983fd2)

Solutions

1. Point the aggregator at a column of type tdigestsketch (MergingDigest values) or a numeric column.
2. If the value is serialized bytes, deserialize it first via TDigestSketchAggregatorFactory.deserialize / the sqlTypeName coercion.
3. Remove or fix the expression/post-aggregator that is emitting an unexpected type.
4. Check the input column's actual types with a small sample query before aggregating.

Example fix

// before
new TDigestSketchAggregatorFactory("sk", "someStringColumn", 100);
// after
new TDigestSketchAggregatorFactory("sk", "tdigestColumn", 100);

if (!(v instanceof Number || v instanceof MergingDigest)) { throw new IllegalArgumentException("column must be numeric or tdigestsketch, got: " + v.getClass()); }

boolean isAggregateable(Object v) { return v instanceof Number || v instanceof MergingDigest; }

try { agg.aggregate(selector); } catch (IAE e) { log.error("bad input to tdigest aggregator: %s", e.getMessage()); }

When it happens

Trigger: Feeding the tdigestsketch aggregator a column or expression that yields non-numeric, non-MergingDigest objects, e.g. string fields, HyperLogLog sketches, arrays, or serialized (non-deserialized) sketch representations.

Common situations: Pointing the aggregator at a raw string/complex column instead of a tdigestsketch column; a post-aggregator or expression outputting the wrong type; mixing aggregation on deserialized vs serialized sketch objects.

Understand the failure class

Background: Type mismatch errors: IllegalArgumentException, TypeError and type guards across 150 open-source libraries — this error's family across 150 libraries.

Related errors

• Cannot deserialize type[%s] to an RoaringBitmap64Counter:
• Casting to float type is not supported
• Casting to long type is not supported
• AggregatorFactoryNotMergeableException
• Incompatible type for metric[%s], expected a ApproximateHist

AI-assisted analysis of apache/druid@9b90983fd2 (2026-09-07). Data as JSON: /api/errors/fa4c78adedea5a11. Report an issue: GitHub.