apache/skywalking · error · IllegalExpressionException

Single Labeled to Single Labeled, right labeled result has m

Error message

Single Labeled to Single Labeled, right labeled result has more than one value.

What it means

In LROp.single2SingleLabeled (both operands labeled SINGLE_VALUE), the right operand's label groups are indexed into a map keyed by the HashSet of labels. When a left label group finds a matching right group, that right group must contain exactly one MQEValue; otherwise this exception is thrown because the pairing is ambiguous.

Source

Thrown at oap-server/mqe-rt/src/main/java/org/apache/skywalking/mqe/rt/operation/LROp.java:142

    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);
            }
        }

        return singleLeft;
    }

    //series or list or labeled single value with scalar
    private static ExpressionResult many2OneBinaryOp(ExpressionResult manyResult,
                                                     ExpressionResult singleResult,
                                                     int opType, LROp calculate) throws IllegalExpressionException {

View on GitHub (pinned to 102af09b4a)

Solutions

1. Query the right operand alone and check that every label group matching a left label has exactly one value
2. Tighten the relabels rules on the right side so each label combination maps to a single value
3. Aggregate the right operand (sum/avg/latest) before applying labels
4. Drop extra labels on the right side so groups do not fold multiple values together

Example fix

// before
query: relabels(sum(a), tag:(k,v)) + relabels(b, tag:(k,v))  // b has 2 values for (k,v)
// after
query: relabels(sum(a), tag:(k,v)) + relabels(sum(b), tag:(k,v))

Map<Set<KeyValue>, Integer> sizes = right.getResults().stream()
    .collect(toMap(g -> new HashSet<>(g.getMetric().getLabels()), g -> g.getValues().size()));
boolean ok = sizes.values().stream().allMatch(s -> s == 1);

catch (IllegalExpressionException e) { surfaceQueryError(e); }

When it happens

Trigger: Both sides of a binary op are labeled scalars and share at least one label combination whose RIGHT-side group contains 2+ values. Non-matching label groups do NOT throw — they set emptyValue on the left — so the error only fires on matched labels with multi-valued right groups.

Common situations: relabels collapsing multiple right-side series into one label group that then matches a left label; a labeled right metric whose OAL/MAL definition emits several values per label set after a config change; mixing a labeled scalar with a metric that still carries extra labels that were meant to be dropped.

Related errors

• Single Labeled to Single Labeled, left labeled result is emp
• 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/723c4a0a7e0fc248. Report an issue: GitHub.