apache/druid · error · IllegalStateException

Unsupported nested type: [%s]

Error message

Unsupported nested type: [%s]

What it means

FieldTypeInfo.add() builds the set of ColumnTypes encoded in a nested field's serialized type bitmap byte. Each byte value maps to a scalar or array type; any unmapped byte means the segment was written by a newer Druid version (or is corrupt) whose type code this version cannot interpret, so it throws an ISE rather than guessing.

Source

Thrown at processing/src/main/java/org/apache/druid/segment/nested/FieldTypeInfo.java:320

        break;
      case ARRAY:
        Preconditions.checkNotNull(type.getElementType(), "ElementType must not be null");
        switch (type.getElementType().getType()) {
          case STRING:
            types |= STRING_ARRAY_MASK;
            break;
          case LONG:
            types |= LONG_ARRAY_MASK;
            break;
          case DOUBLE:
            types |= DOUBLE_ARRAY_MASK;
            break;
          default:
            throw new ISE("Unsupported nested array type: [%s]", type.asTypeString());
        }
        break;
      default:
        throw new ISE("Unsupported nested type: [%s]", type.asTypeString());
    }
    return types;
  }

  public static Set<ColumnType> convertToSet(byte types)
  {
    final Set<ColumnType> theTypes = Sets.newHashSetWithExpectedSize(4);
    if ((types & STRING_MASK) > 0) {
      theTypes.add(ColumnType.STRING);
    }
    if ((types & LONG_MASK) > 0) {
      theTypes.add(ColumnType.LONG);
    }
    if ((types & DOUBLE_MASK) > 0) {
      theTypes.add(ColumnType.DOUBLE);
    }
    if ((types & STRING_ARRAY_MASK) > 0) {
      theTypes.add(ColumnType.STRING_ARRAY);

View on GitHub (pinned to 9b90983fd2)

Solutions

1. Upgrade Druid to a version that supports the nested type code in the segment (check the version that wrote the segment).
2. Re-ingest the data with the older/compatible writer version to regenerate the column type metadata.
3. Verify segment integrity; re-download or re-restore the segment if bytes are corrupted.
4. If downgrading is intentional, drop or re-ingest columns using new nested types before downgrading.

Example fix

// before (older Druid reading newer segment)
// ISE: Unsupported nested type: [ARRAY<COMPLEX>]
// after: upgrade the cluster so reader version >= writer version,
// or re-ingest with the older version's supported types

// before reading nested columns, check writer vs reader support
ColumnTypeInfo info = ColumnTypeInfo.getMetadata(colMetadata);
// or: verify cluster version >= version that wrote the segment
if (!SUPPORTED_NESTED_TYPES.contains(typeCode)) { throw new SkipSegmentException(...); }

boolean isReadableNestedType(byte typeCode) {
  return ColumnTypeInfo.values().length > 0 && java.util.Arrays.stream(ColumnTypeInfo.values())
      .anyMatch(t -> t.getType() == typeCode);
}

try {
  types = FieldTypeInfo.add(existing, type);
} catch (IllegalStateException e) {
  if (e.getMessage().startsWith("Unsupported nested")) {
    log.warn(e, "Segment uses unsupported nested type; skipping/upgrade required");
  } else throw e;
}

When it happens

Trigger: Reading a nested column (auto-type column / nested data format) whose serialized type-byte encodes a type (or nested-array type) unsupported by the running Druid version; corrupted type metadata bytes.

Common situations: Rolling upgrade/downgrade across Druid versions where a newer writer produced V4+ nested columns with new type codes; reading segments from a fork or patched build with custom type codes; segment file corruption.

Related errors

• Cannot merge columns of type[%s] and format[%s] and with [%s
• Unknown version
• Unknown version
• not a scalar in the dictionary
• Cannot deserialize type[%s] to an RoaringBitmap64Counter:

AI-assisted analysis of apache/druid@9b90983fd2 (2026-09-07). Data as JSON: /api/errors/d54714a1e44317a0. Report an issue: GitHub.