apache/skywalking · error · IllegalExpressionException

Bool Operation: The result of the left expression is not a c

Error message

Bool Operation: The result of the left expression is not a compare result.

What it means

IllegalExpressionException from BoolOp.doBoolOp: boolean connectors (&&, ||) in MQE operate on the results of comparison expressions, and the left operand's ExpressionResult.isBoolResult() is false — i.e. the left side is a plain metric value, not the outcome of a compare operation (>, >=, ==, etc.).

Source

Thrown at oap-server/mqe-rt/src/main/java/org/apache/skywalking/mqe/rt/operation/BoolOp.java:30

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package org.apache.skywalking.mqe.rt.operation;

import org.apache.skywalking.mqe.rt.exception.IllegalExpressionException;
import org.apache.skywalking.mqe.rt.grammar.MQEParser;
import org.apache.skywalking.oap.server.core.query.mqe.ExpressionResult;

public class BoolOp {
    public static ExpressionResult doBoolOp(ExpressionResult left,
                                            ExpressionResult right,
                                            int opType) throws IllegalExpressionException {
        if (!checkExpression(left)) {
            throw new IllegalExpressionException(
                "Bool Operation: The result of the left expression is not a compare result.");
        }

        if (!checkExpression(right)) {
            throw new IllegalExpressionException(
                "Bool Operation: The result of the right expression is not a compare result.");
        }

        try {
            return LROp.doLROp(left, right, opType, BoolOp::boolOp);
        } catch (IllegalExpressionException e) {
            throw new IllegalExpressionException("Unsupported bool operation: " + e.getMessage());
        }
    }

    //bool with bool
    private static double boolOp(double leftValue, double rightValue, int opType) throws IllegalExpressionException {
        // this should not happen, but just in case

View on GitHub (pinned to 102af09b4a)

Solutions

1. Wrap the left operand in an explicit comparison: metric > 0, metric >= threshold
2. Make every leaf of a && / || chain a compare expression ('a > b && c < d')
3. Check for accidental operator omission when concatenating conditions in generated dashboards

Example fix

# before
service_sla && service_resp_time > 500
# after
service_sla > 0 && service_resp_time > 500

// every leaf of && / || must be a compare result
boolean isBoolLeaf(ExpressionResult r) { return r.isBoolResult(); }

boolean isCompareResult(org.apache.skywalking.oap.server.core.query.mqe.ExpressionResult r) {
    return r.isBoolResult();
}

When it happens

Trigger: An MQE expression like 'service_sla > 1000 && service_resp_time > 500' is fine, but 'service_sla && service_resp_time > 500' (left side never compared) triggers it: the left sub-result was not produced by CompareOp so it carries no boolean semantics.

Common situations: Forgetting the comparison on one side of a logical expression; using a boolean-typed metric name directly expecting truthiness; copying SQL/PromQL boolean-idioms into MQE where only explicit comparisons yield booleans.

Related errors

• Bool Operation: The result of the right expression is not a
• Unsupported bool operation: {}
• Unsupported compare operation: {}
• Unsupported predicted value type: {}
• line {}:{} {}

AI-assisted analysis of apache/skywalking@102af09b4a (2026-08-14). Data as JSON: /api/errors/5d6577b1dcf9223e. Report an issue: GitHub.