app/vmselect: return metric values from time() cmp_op metric query when cmp_op comparison is true

This aligns MetricsQL behavior to Prometheus' one.

The issue has been identified at https://promlabs.com/promql-compliance-test-results/2020-12-01/victoriametrics/

app/vmselect/promql/exec.go

@@ -149,11 +149,11 @@ func adjustCmpOps(e metricsql.Expr) metricsql.Expr {
 		if !metricsql.IsBinaryOpCmp(be.Op) {
 			return
 		}
-		if _, ok := be.Left.(*metricsql.NumberExpr); !ok {
+		if isNumberExpr(be.Right) || !isScalarExpr(be.Left) {
 			return
 		}
 		// Convert 'num cmpOp query' expression to `query reverseCmpOp num` expression
-		// like Prometheus does. For isntance, `0.5 < foo` must be converted to `foo > 0.5`
+		// like Prometheus does. For instance, `0.5 < foo` must be converted to `foo > 0.5`
 		// in order to return valid values for `foo` that are bigger than 0.5.
 		be.Right, be.Left = be.Left, be.Right
 		be.Op = getReverseCmpOp(be.Op)
@@ -161,6 +161,22 @@ func adjustCmpOps(e metricsql.Expr) metricsql.Expr {
 	return e
 }
 
+func isNumberExpr(e metricsql.Expr) bool {
+	_, ok := e.(*metricsql.NumberExpr)
+	return ok
+}
+
+func isScalarExpr(e metricsql.Expr) bool {
+	if isNumberExpr(e) {
+		return true
+	}
+	if fe, ok := e.(*metricsql.FuncExpr); ok {
+		// time() returns scalar in PromQL - see https://prometheus.io/docs/prometheus/latest/querying/functions/#time
+		return strings.ToLower(fe.Name) == "time"
+	}
+	return false
+}
+
 func getReverseCmpOp(op string) string {
 	switch op {
 	case ">":

docs/CHANGELOG.md

@@ -7,6 +7,7 @@
 * BUGFIX: return `nan` for `a >bool b` query when `a` equals to `nan` like Prometheus does. Previously `0` was returned in this case. This applies to any comparison operation
   with `bool` modifier. See [these docs](https://prometheus.io/docs/prometheus/latest/querying/operators/#comparison-binary-operators) for details.
 * BUGFIX: properly parse hex numbers in MetricsQL. Previously hex numbers with non-decimal digits such as `0x3b` couldn't be parsed.
+* BUGFIX: Handle `time() cmp_op metric` like Prometheus does - i.e. return `metric` value if `cmp_op` comparison is true. Previously `time()` value was returned.
 
 
 # [v1.48.0](https://github.com/VictoriaMetrics/VictoriaMetrics/releases/tag/v1.48.0)