{"record":{"id":"4da5c65d40548a05","repo":"apache/flink","slug":"cannot-use-the-key-partitioner-for-composite-keys-4da5c6","errorCode":null,"errorMessage":"Cannot use the key partitioner for composite keys (more than one key field)","messagePattern":"Cannot use the key partitioner for composite keys \\(more than one key field\\)","errorType":"validation","errorClass":"IllegalArgumentException","httpStatus":null,"severity":"error","filePath":"flink-core/src/main/java/org/apache/flink/api/common/operators/base/ReduceOperatorBase.java","lineNumber":189,"sourceCode":"     *     function.\n     * @param name The name of the operator (for logging and messages).\n     */\n    public ReduceOperatorBase(\n            Class<? extends FT> udf, UnaryOperatorInformation<T, T> operatorInfo, String name) {\n        super(new UserCodeClassWrapper<FT>(udf), operatorInfo, name);\n    }\n\n    // --------------------------------------------------------------------------------------------\n\n    public void setCustomPartitioner(Partitioner<?> customPartitioner) {\n        if (customPartitioner != null) {\n            int[] keys = getKeyColumns(0);\n            if (keys == null || keys.length == 0) {\n                throw new IllegalArgumentException(\n                        \"Cannot use custom partitioner for a non-grouped GroupReduce (AllGroupReduce)\");\n            }\n            if (keys.length > 1) {\n                throw new IllegalArgumentException(\n                        \"Cannot use the key partitioner for composite keys (more than one key field)\");\n            }\n        }\n        this.customPartitioner = customPartitioner;\n    }\n\n    public Partitioner<?> getCustomPartitioner() {\n        return customPartitioner;\n    }\n\n    // --------------------------------------------------------------------------------------------\n\n    @Override\n    protected List<T> executeOnCollections(\n            List<T> inputData, RuntimeContext ctx, ExecutionConfig executionConfig)\n            throws Exception {\n        // make sure we can handle empty inputs\n        if (inputData.isEmpty()) {","sourceCodeStart":171,"sourceCodeEnd":207,"githubUrl":"https://github.com/apache/flink/blob/2f3c205e9266cb30240eb7f4fdab15cad629a70f/flink-core/src/main/java/org/apache/flink/api/common/operators/base/ReduceOperatorBase.java#L171-L207","documentation":"Error \"Cannot use the key partitioner for composite keys (more than one key field)\" thrown in apache/flink.","triggerScenarios":"Triggered at runtime when the operation fails because: Cannot use the key partitioner for composite keys (more than one key field).","commonSituations":"Commonly caused by misconfiguration, missing dependencies or files, unsupported types or operations, or invalid user input leading to: Cannot use the key partitioner for composite keys (more than one key field).","solutions":["Address the cause reported by the error message: Cannot use the key partitioner for composite keys (more than one key field)","Verify the inputs, configuration values, and classpath/dependency setup related to this operation, then retry."],"exampleFix":"Correct the condition described (\"Cannot use the key partitioner for composite keys (more than one key field)\") and rerun the job or command.","handlingStrategy":null,"validationCode":null,"typeGuard":null,"tryCatchPattern":null,"preventionTips":[],"tags":[],"backgroundTag":null,"analyzedSha":"2f3c205e9266cb30240eb7f4fdab15cad629a70f","analyzedAt":"2026-08-14T08:48:24.518Z","schemaVersion":2},"datasetVersion":"2026-08-14T10:17:34.591Z"}