What causes "Not decorated" in apache/druid?

Run the post-aggregator through the normal query decoration/finalization pipeline so decorate(...) installs the comparator

apache/druid · error · UnsupportedOperationException

Not decorated

Error message

Not decorated

What it means

FinalizingFieldAccessPostAggregator.getComparator throws UnsupportedOperationException("Not decorated") when the aggregator has not been wrapped by the finalization decorator that installs the comparator. This class is a shell: its comparator/finalizer fields are populated by decorate(...), and direct use of the undecorated instance is a programming error.

Solutions

Run the post-aggregator through the normal query decoration/finalization pipeline so decorate(...) installs the comparator
If decorating manually, call decorate(...) with the appropriate input before invoking getComparator()
Use the factory's finalizeComputation path instead of accessing the raw comparator of an undecorated instance

Example fix

// before
Comparator c = new FinalizingFieldAccessPostAggregator("x", "agg").getComparator();
// after
PostAggregator decorated = new FinalizingFieldAccessPostAggregator("x", "agg")
    .decorate(Collections.singletonList(aggregatorFactory));
Comparator c = decorated.getComparator();

Defensive patterns

Strategy: validation

Validate before calling

if (postAgg instanceof FinalizingFieldAccessPostAggregator && !isDecorated(postAgg)) {
  throw new IllegalStateException("decorate() before getComparator()");
}

Type guard

boolean isDecorated(FinalizingFieldAccessPostAggregator p) { try { p.getComparator(); return true; } catch (UnsupportedOperationException e) { return false; } }

Try / catch

try { c = postAgg.getComparator(); } catch (UnsupportedOperationException e) { c = defaultComparatorForField(fieldName); }

Prevention

Always let the query engine's decoration phase wrap post-aggregators
In tests, call decorate(...) with real AggregatorFactories before asserting
Never call getComparator/compute on raw FinalizingFieldAccessPostAggregator instances

When it happens

Trigger: Calling getComparator() on a FinalizingFieldAccessPostAggregator that was never passed through AggregatorFactory's finalization/decoration step, or on one whose delegate produced a null comparator.

Common situations: Unit tests instantiating the post-aggregator directly without decorating it; custom query code building post-aggregators manually and skipping the decoration phase of query processing.

Related errors

AI-assisted analysis of apache/druid@9b90983fd2 (2026-09-07). Data as JSON: /api/errors/edc335d8707272d4. Report an issue: GitHub.

Appendix: source

Thrown at processing/src/main/java/org/apache/druid/query/aggregation/post/FinalizingFieldAccessPostAggregator.java:86

  {
    this.name = name;
    this.fieldName = fieldName;
    this.finalizedType = finalizedType;
    this.comparator = comparator;
    this.finalizer = finalizer;
  }

  @Override
  public Set<String> getDependentFields()
  {
    return Sets.newHashSet(fieldName);
  }

  @Override
  public Comparator getComparator()
  {
    if (comparator == null) {
      throw new UnsupportedOperationException("Not decorated");
    } else {
      return comparator;
    }
  }

  @Override
  public Object compute(Map<String, Object> combinedAggregators)
  {
    if (finalizer == null) {
      throw new UnsupportedOperationException("Not decorated");
    } else {
      return finalizer.apply(combinedAggregators.get(fieldName));
    }
  }

  @Override
  @JsonProperty
  public String getName()

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