apache/druid · error · org.apache.druid.java.util.common.ISE

Illegal type received while theta sketch merging [%s]

Error message

Illegal type received while theta sketch merging [%s]

What it means

SketchAggregator.updateUnion accepts theta sketch objects, Memory, or Strings (hex/base64) for merging; any other input type triggers this IllegalStateException. It indicates the aggregation is receiving a value that is not a theta sketch in any accepted representation.

Source

Thrown at extensions-core/datasketches/src/main/java/org/apache/druid/query/aggregation/datasketches/theta/SketchAggregator.java:160

    } else if (update instanceof Double) {
      union.update((Double) update);
    } else if (update instanceof Integer || update instanceof Long) {
      union.update(((Number) update).longValue());
    } else if (update instanceof int[]) {
      union.update((int[]) update);
    } else if (update instanceof long[]) {
      union.update((long[]) update);
    } else if (update instanceof List) {
      for (Object entry : (List) update) {
        if (entry != null) {
          final String asString = entry.toString();
          if (asString != null) {
            union.update(asString);
          }
        }
      }
    } else {
      throw new ISE("Illegal type received while theta sketch merging [%s]", update.getClass());
    }
  }

  /**
   * Gets the initial size of this aggregator in bytes.
   */
  public long getInitialSizeBytes()
  {
    // Size = 16B (object header) + 24B (3 refs) + 4B (int size) = 44B
    // Due to 8-byte alignment, size = 48B
    // (see https://www.baeldung.com/java-memory-layout)
    return 48L;
  }

}

View on GitHub (pinned to 9b90983fd2)

Solutions

Ensure the column is a theta sketch (thetaSketch aggregator at ingestion); re-ingest or convert the metric if it is another sketch type
Check that queries do not mix sketch types across segments/subqueries
If input is serialized data, ensure it is provided as String (hex/base64), Memory, or Sketch

Example fix

// before
String payload = readFieldAsObject(row); // returns HLL object
union.update(payload); // ISE
// after
ThetaValue v = toThetaSketch(row.getField("sketch")); // re-ingest column as thetaSketch if not convertible

Defensive patterns

Strategy: validation

Validate before calling

Object v = selector.get(); if (!(v instanceof Sketch || v instanceof Memory || v instanceof String)) { throw new IllegalArgumentException("Column must be thetaSketch, got " + v.getClass()); }

Type guard

boolean isMergeableTheta(Object v) { return v instanceof Sketch || v instanceof Memory || v instanceof String; }

Try / catch

try { agg.aggregate(); } catch (IllegalStateException e) { if (e.getMessage().startsWith("Illegal type received while theta sketch merging")) { log.error("Re-ingest column as thetaSketch"); } else throw e; }

Prevention

Ingest theta sketch columns with the thetaSketch aggregator
Never mix sketch families (HLL, quantiles) in one metric column
Verify column types across all segments before querying

When it happens

Trigger: aggregate()/aggregateWithSize() feed updateUnion a selector value whose runtime class is neither Sketch, Memory, nor String — commonly because the rolled-up metric object is of a different sketch family (HLL, quantiles) or a deserialized generic Object.

Common situations: Querying a theta sketch column that was ingested with the wrong sketch type (e.g. HLLSketch metrics); mixing sketch families in a datasource; segment/rollup type drift after schema changes.

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

AI-assisted analysis of apache/druid@9b90983fd2 (2026-09-07). Data as JSON: /api/errors/c87f023350a920a9. Report an issue: GitHub.