apache/druid · error · IllegalArgumentException

Expected a number or an instance of DDSketch, but received [

Error message

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

What it means

DDSketchAggregator.aggregate() only accepts input values that are Numbers or existing DDSketch sketches; anything else (e.g. String, null-like sentinel objects) throws this IAE. It typically indicates the selector/field expression produced an unexpected type, or sketches were combined across incompatible columns.

Source

Thrown at extensions-contrib/ddsketch/src/main/java/org/apache/druid/query/aggregation/ddsketch/DDSketchAggregator.java:68

    double effectiveRelativeError = relativeError != null ? relativeError : DDSketchAggregatorFactory.DEFAULT_RELATIVE_ERROR;
    this.selector = selector;
    this.histogram = DDSketches.collapsingLowestDense(effectiveRelativeError, effectiveNumBins);
  }

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

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

  @Override
  public float getFloat()
  {

View on GitHub (pinned to 9b90983fd2)

Solutions

  1. Point the aggregator's fieldName at a numeric column or a column holding DDSketch objects.
  2. Add an expression selector that casts the value to a number, e.g. type 'expression' with expression "CAST(\"col\" AS DOUBLE)" or "atoi(col)".
  3. Verify the upstream column type with a small query before wiring the aggregation.
  4. If aggregating existing sketches, ensure the source was produced by ddsketch, not another sketch type (e.g. HLL, quantiles).

Example fix

// before
{"type": "ddsketch", "fieldName": "tags"} // tags is a string dimension
// after
{"type": "ddsketch", "fieldExpression": {"type": "expression", "expression": "CAST(\"value\" AS DOUBLE)"}}
Defensive patterns

Strategy: type-guard

Validate before calling

// verify the input column type via segment metadata query before configuring the aggregator
// SELECT column, dataType FROM sys.segments WHERE ... (inspect dataType is NUMERIC or COMPLEX<ddsketch>)

Type guard

Object v = selector.get();
if (!(v instanceof Number) && !(v instanceof DDSketch)) {
  throw new IllegalArgumentException("ddsketch input must be Number or DDSketch, got " + (v == null ? "null" : v.getClass()));
}

Try / catch

try {
  agg.aggregate(val);
} catch (IllegalArgumentException e) {
  log.warn(e, "skipping non-numeric value for ddsketch: %s", val);
}

Prevention

When it happens

Trigger: Aggregating on a column whose selector returns a non-numeric object (e.g. String from a dimension), applying the ddsketch aggregator to a field that yields an unsupported type, or feeding a post-aggregator of the wrong type into another ddsketch aggregator.

Common situations: Wrong fieldName pointing at a string dimension instead of a numeric/sketch column, using ddsketch on nested/array values, or mismatched input/output types in a complex aggregation chain.

Understand the failure class

Background: "Must be a positive integer", "Invalid value", "Unsupported": the invalid-argument-value error family, when a library rejects the value you pass — this error's family across 35 libraries.

Related errors

AI-assisted analysis of apache/druid@9b90983fd2 (2026-09-07). Data as JSON: /api/errors/d43801933f7caa2e. Report an issue: GitHub.