apache/seatunnel · error · UnsupportedOperationException

List as map key is not supported

Error message

List as map key is not supported

What it means

ListTypeWriter.writeToMapKey is a placeholder in the Lance sink's Arrow type-writer dispatch. Arrow Map key vectors only support scalar key types, so writing a SeaTunnel ARRAY value into a map KEY slot is rejected outright. The method intentionally throws UnsupportedOperationException because there is no valid Arrow encoding for this operation.

Solutions

1. Change the map key type in the SeaTunnel schema to a scalar type (STRING, INT, etc.) — Arrow does not allow list keys.
2. If a composite key is needed, serialize the list into a single delimited string (e.g. JSON or join) and use STRING as the map key.
3. Restructure the data as a list of structs (LIST<STRUCT<key-fields, value>>) instead of a map with composite keys.
4. Track the Lance connector for future support; the surrounding methods are marked 'not yet implemented'.

Example fix

// before: schema with map key as array
columns = "info map<array<string>, string>"

// after: use scalar key
columns = "info map<string, string>"

// Java: before writing, check map key type is scalar
if (mapFieldType.getChildren().get(0).getType() instanceof ArrowType.List) {
    throw new IllegalArgumentException("map key cannot be a list; use a scalar key type");
}

boolean isScalarArrowType(ArrowType t) {
    return t instanceof ArrowType.Int || t instanceof ArrowType.FloatingPoint
        || t instanceof ArrowType.Utf8 || t instanceof ArrowType.Binary
        || t instanceof ArrowType.Bool || t instanceof ArrowType.Date
        || t instanceof ArrowType.Time || t instanceof ArrowType.TimeStamp;
}

When it happens

Trigger: A SeaTunnel Lance sink row contains a MAP whose key type is defined as an ARRAY (list) type, and FragmentConverter dispatches the key value through UnionMapWriter, which routes to ListTypeWriter.writeToMapKey.

Common situations: Declaring a table schema in the job config where the map key column type is arrays instead of a scalar (string/int); upstream data pipelines that model composite keys as lists; schema evolution mistakes when the key field was changed to a nested type.

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

• Map as map key is not supported
• Unsupported ArrowType
• List as map value not yet implemented
• Map as map value not yet implemented
• Map in list writing not yet implemented

AI-assisted analysis of apache/seatunnel@cf67b549a7 (2026-09-10). Data as JSON: /api/errors/722192d9a4cd6597. Report an issue: GitHub.

Appendix: source

Thrown at seatunnel-connectors-v2/connector-lance/src/main/java/org/apache/seatunnel/connectors/seatunnel/lance/sink/writers/ListTypeWriter.java:48

            FieldVector vector,
            ArrowType arrowType,
            Object value,
            int rowIndex,
            BufferAllocator allocator) {
        throw new UnsupportedOperationException(
                "List type should be handled via FragmentConverter.writeListToVector");
    }

    @Override
    public void writeToListWriter(
            UnionListWriter writer, ArrowType arrowType, Object value, BufferAllocator allocator) {
        throw new UnsupportedOperationException("Nested list writing not yet implemented");
    }

    @Override
    public void writeToMapKey(
            UnionMapWriter writer, ArrowType arrowType, Object value, BufferAllocator allocator) {
        throw new UnsupportedOperationException("List as map key is not supported");
    }

    @Override
    public void writeToMapValue(
            UnionMapWriter writer, ArrowType arrowType, Object value, BufferAllocator allocator) {
        throw new UnsupportedOperationException("List as map value not yet implemented");
    }
}

View on GitHub (pinned to cf67b549a7)