apache/druid · error · IllegalArgumentException

Cannot make array element selector for path

Error message

Cannot make array element selector for path [%s], negative array index not supported for this selector

What it means

When a nested field path references an array element (NestedArrayElement) with a negative element number, makeDimensionSelector cannot build the selector and throws IAE. Negative array indices are not supported for dimension selectors on nested columns.

Solutions

1. Use a non-negative array index in the path (e.g. $.arr[0])
2. To get the last element, use SQL functions like ARRAY_OFFSET/ARRAY_SLICE or reverse logic instead of -1
3. Validate elementNumber >= 0 before constructing NestedArrayElement

Example fix

// before
String path = "$.tags[-1]";
// after
String path = "$.tags[0]"; // or compute size-1 via array functions

int idx = nestedField.elementNumber; if (idx < 0) { throw new IllegalArgumentException("Negative array index not supported, use >= 0: " + path); }

boolean hasPositiveArrayIndex(String jsonPath) { return !java.util.regex.Pattern.compile("\\[\\s*-\\d+\\s*\\]").matcher(jsonPath).find(); }

try { sel = column.makeDimensionSelector(offset, extractionFn); } catch (IAE e) { log.warn("Unsupported path %s: %s", path, e.getMessage()); sel = null; }

When it happens

Trigger: Calling makeDimensionSelector (via fieldSelector) with a path like $.arr[-1] or JSON_VALUE using a negative array index; NestedFieldPath parsed to NestedArrayElement with elementNumber < 0.

Common situations: Users writing SQL with negative JSON array indexing (e.g. '$[-1]') expecting Python-like last-element semantics; programmatically built NestedFieldPath with elementNumber computed to a negative value.

Understand the failure class

Background: UnsupportedOperationException and "is not supported" errors: when a library deliberately refuses a call — this error's family across 30 libraries.

Related errors

• Cannot use lookupNameUtf8 on this selector
• Multi-value row not supported
• Unsupported nested array type
• Aggregator[ ] cannot vectorize
• AppenderatorsManager methods should only called by services…

AI-assisted analysis of apache/druid@9b90983fd2 (2026-09-07). Data as JSON: /api/errors/359b4e1eb9cd2a88. Report an issue: GitHub.

Appendix: source

Thrown at processing/src/main/java/org/apache/druid/segment/nested/CompressedNestedDataComplexColumn.java:580

  public DimensionSelector makeDimensionSelector(
      List<NestedPathPart> path,
      ExtractionFn extractionFn,
      ColumnSelectorFactory selectorFactory,
      ReadableOffset readableOffset
  )
  {
    final Field field = getNestedFieldOrNestedArrayElementFromPath(path);
    if (field instanceof NestedField) {
      DictionaryEncodedColumn<?> col = (DictionaryEncodedColumn<?>) getColumnHolder(
          ((NestedField) field).fieldName,
          ((NestedField) field).fieldIndex
      ).getColumn();
      return col.makeDimensionSelector(readableOffset, extractionFn);
    } else if (field instanceof NestedArrayElement) {
      final NestedArrayElement arrayField = (NestedArrayElement) field;
      final int elementNumber = arrayField.elementNumber;
      if (elementNumber < 0) {
        throw new IAE(
            "Cannot make array element selector for path [%s], negative array index not supported for this selector",
            path
        );
      }
      ColumnValueSelector<?> arraySelector = getColumnHolder(
          arrayField.nestedField.fieldName,
          arrayField.nestedField.fieldIndex
      ).getColumn().makeColumnValueSelector(readableOffset);
      return new BaseSingleValueDimensionSelector()
      {
        @Nullable
        @Override
        protected String getValue()
        {
          Object o = arraySelector.getObject();
          if (o instanceof Object[]) {
            Object[] array = (Object[]) o;
            if (elementNumber < array.length) {

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