prestodb/presto · error · UnsupportedOperationException

LEARN must run on a single machine

Error message

LEARN must run on a single machine

What it means

Unsupported-operation guard in LearnRegressorAggregation.combine: the base learn_regressor aggregation delegates to the LibSVM variant but still must not merge partial states — the aggregation is declared decomposable=false, and combine is only a placeholder that rejects distributed execution of the training.

Source

Thrown at presto-ml/src/main/java/com/facebook/presto/ml/LearnRegressorAggregation.java:56

            @SqlType(BIGINT) long label,
            @SqlType("map(bigint,double)") Block features)
    {
        input(state, (double) label, features);
    }

    @InputFunction
    public static void input(
            @AggregationState LearnState state,
            @SqlType(DOUBLE) double label,
            @SqlType("map(bigint,double)") Block features)
    {
        LearnLibSvmRegressorAggregation.input(state, label, features, Slices.utf8Slice(""));
    }

    @CombineFunction
    public static void combine(@AggregationState LearnState state, @AggregationState LearnState otherState)
    {
        throw new UnsupportedOperationException("LEARN must run on a single machine");
    }

    @OutputFunction(RegressorType.NAME)
    public static void output(@AggregationState LearnState state, BlockBuilder out)
    {
        LearnLibSvmRegressorAggregation.output(state, out);
    }
}

View on GitHub (pinned to 55bb57d202)

Solutions

1. Run learn_regressor on a single node / single split
2. Ensure no partial-aggregation plan is generated for this function
3. Reduce input to one worker before training

When it happens

Trigger: Thrown at presto-ml/src/main/java/com/facebook/presto/ml/LearnRegressorAggregation.java:56 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of prestodb/presto@55bb57d202 (2026-09-04). Data as JSON: /api/errors/838624eb91df8498. Report an issue: GitHub.