apache/druid · error · UnsupportedColumnTypeException

Cannot handle column

Error message

Cannot handle column [%s] with unknown type

What it means

FrameColumnWriters.create builds a column writer for the given ColumnType. A null type means the RowSignature never assigned a type to the column, so no writer can be chosen; it throws UnsupportedColumnTypeException(column, null) immediately. This fails fast at frame-writing time for untyped columns.

Solutions

1. Give every column an explicit type in the RowSignature or ingestion spec
2. CAST untyped expressions in the SQL (e.g. CAST(x AS BIGINT)) before they reach frame writing
3. Fix schema-inference code so it never yields null ColumnTypes

Example fix

// before
.add("nullableCol", null)
// after
.add("nullableCol", ColumnType.STRING)

Preconditions.checkNotNull(signature.getColumnType(colIdx).orElse(null), "Column %s must be typed", column);

boolean writableColumnType(ColumnType t) { return t != null; }

try { FrameColumnWriters.create(...); } catch (UnsupportedColumnTypeException e) { /* assign a type and rebuild the writer */ }

When it happens

Trigger: Calling FrameColumnWriters.create (via FrameWriterFactory/FrameWriterUtils pipelines) with a signature column whose ColumnType is null — e.g. a signature built from inferred or external schema without a type.

Common situations: Ingestion/MSQ specs selecting untyped expressions (NULL literals, untyped nested queries); programmatic RowSignature building omitting types; schema inference returning nothing for empty input.

Understand the failure class

Background: Schema validation failed / invalid input schema: payload rejected because its shape doesn't match the expected schema — this error's family across 28 libraries.

Related errors

• Cannot handle column
• Cannot handle column
• Cannot create comparator for array type
• Cannot handle column
• Cannot handle column

AI-assisted analysis of apache/druid@9b90983fd2 (2026-09-07). Data as JSON: /api/errors/cbf20db80f95a0bf. Report an issue: GitHub.

Appendix: source

Thrown at processing/src/main/java/org/apache/druid/frame/write/columnar/FrameColumnWriters.java:67

  private FrameColumnWriters()
  {
    // No instantiation.
  }

  /**
   * Helper used by {@link ColumnarFrameWriterFactory}.
   *
   * @throws UnsupportedColumnTypeException if "type" cannot be handled
   */
  static FrameColumnWriter create(
      final ColumnSelectorFactory columnSelectorFactory,
      final MemoryAllocator allocator,
      final String column,
      final ColumnType type
  )
  {
    if (type == null) {
      throw new UnsupportedColumnTypeException(column, null);
    }

    switch (type.getType()) {
      case LONG:
        return makeLongWriter(columnSelectorFactory, allocator, column);
      case FLOAT:
        return makeFloatWriter(columnSelectorFactory, allocator, column);
      case DOUBLE:
        return makeDoubleWriter(columnSelectorFactory, allocator, column);
      case STRING:
        return makeStringWriter(columnSelectorFactory, allocator, column);
      case ARRAY:
        switch (type.getElementType().getType()) {
          case STRING:
            return makeStringArrayWriter(columnSelectorFactory, allocator, column);
          case LONG:
            return makeLongArrayWriter(columnSelectorFactory, allocator, column);
          case FLOAT:

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