apache/skywalking · error · IllegalExpressionException

Single Labeled to Single Labeled, left labeled result is emp

Error message

Single Labeled to Single Labeled, left labeled result is empty or has more than one value.

What it means

Thrown by LROp.single2SingleLabeled, which handles SINGLE_VALUE op SINGLE_VALUE where BOTH sides are labeled results. The left side is iterated per label-group (MQEValues), and each group must hold exactly one MQEValue. If any labeled group on the left has a value count != 1, the labeled scalar is malformed and the exception fires.

Source

Thrown at oap-server/mqe-rt/src/main/java/org/apache/skywalking/mqe/rt/operation/LROp.java:133

        if (left.isEmptyValue() || right.isEmptyValue()) {
            mqeValue.setEmptyValue(true);
        } else {
            double value = calculate.apply(left.getDoubleValue(), right.getDoubleValue(), opType);
            mqeValue.setDoubleValue(value);
        }
        return result;
    }

    private static ExpressionResult single2SingleLabeled(ExpressionResult singleLeft,
                                                           ExpressionResult singleRight,
                                                           int opType, LROp calculate) throws IllegalExpressionException {
        Map<Set<KeyValue>, List<MQEValue>> labelMapR = new HashMap<>();
        singleRight.getResults().forEach(mqeValuesR -> {
            labelMapR.put(new HashSet<>(mqeValuesR.getMetric().getLabels()), mqeValuesR.getValues());
        });
        for (MQEValues mqeValuesL : singleLeft.getResults()) {
            if (mqeValuesL.getValues().size() != 1) {
                throw new IllegalExpressionException("Single Labeled to Single Labeled, left labeled result is empty or has more than one value.");
            }
            //reserve left metric info
            MQEValue valueL = mqeValuesL.getValues().get(0);
            List<MQEValue> mqeValuesR = labelMapR.get(new HashSet<>(mqeValuesL.getMetric().getLabels()));
            if (mqeValuesR == null) {
                valueL.setEmptyValue(true);
            } else {
                if (mqeValuesR.size() != 1) {
                    throw new IllegalExpressionException("Single Labeled to Single Labeled, right labeled result has more than one value.");
                }
                MQEValue valueR = mqeValuesR.get(0);
                if (valueL.isEmptyValue() || valueR.isEmptyValue()) {
                    valueL.setEmptyValue(true);
                    continue;
                }
                double value = calculate.apply(valueL.getDoubleValue(), valueR.getDoubleValue(), opType);
                valueL.setDoubleValue(value);
            }

View on GitHub (pinned to 102af09b4a)

Solutions

1. Query the left operand alone and inspect each results[] element: every labeled group must have exactly one value
2. Fix the relabels rules or the metric definition so each label combination yields a single value
3. Aggregate before labeling: apply sum()/latest() first, then relabels, so each group is a scalar
4. If multiple values per label are legitimate, redesign the query — single2SingleLabeled semantics require one value per label group

Example fix

// before
query: relabels(a, tag:(key1,val1)) + relabels(b, tag:(key1,val1))
// after (aggregate first so each label group is a scalar)
query: relabels(sum(a), tag:(key1,val1)) + relabels(sum(b), tag:(key1,val1))

boolean ok = left.isLabeledResult()
    && left.getResults().stream().allMatch(g -> g.getValues().size() == 1);

boolean isLabeledScalar(ExpressionResult r) {
    return r.getType() == ExpressionResultType.SINGLE_VALUE
        && r.isLabeledResult()
        && r.getResults().stream().allMatch(g -> g.getValues().size() == 1);
}

catch (IllegalExpressionException e) { log.warn("MQE labeled scalar invalid: {}", e.getMessage()); }

When it happens

Trigger: MQE expression like 'relabels(a, ...) + relabels(b, ...)' or two labeled metrics combined with a binary operator, where at least one left label-group contains 0 or 2+ values. Reached when both operands are SINGLE_VALUE and both isLabeledResult() is true (LROp.java:87-88).

Common situations: Using relabels/on labels on a scalar metric that returns multiple values per label combination; label cardinality mismatch after a relabels rule maps several source labels onto one target label group; a metric redefined in OAL/MAL so it now emits multiple values where one used to be guaranteed.

Related errors

• Single Labeled to Single Labeled, right labeled result has m
• Single to Single, left result is empty or has more than one
• Single to Single, right result is empty or has more than one
• Many to One, single result is empty or has more than one val
• One to Many, single result is empty or has more than one val

AI-assisted analysis of apache/skywalking@102af09b4a (2026-08-14). Data as JSON: /api/errors/9dd23468195ef6af. Report an issue: GitHub.