[BUGFIX] PromQL: make sort_by_label stable

Go's sorting functions can re-order equal elements, so the strategy of
sorting by the fallback ordering first does not always work.
Pulling the fallback into the main comparison function is more reliable
and more efficient.

Signed-off-by: Bryan Boreham <bjboreham@gmail.com>

CHANGELOG.md

@@ -18,7 +18,7 @@
 * [ENHANCEMENT] Remote Read client: Enable streaming remote read if the server supports it. #11379
 * [ENHANCEMENT] Remote-Write: Don't reshard if we haven't successfully sent a sample since last update. #14450
 * [ENHANCEMENT] PromQL: Delay deletion of `__name__` label to the end of the query evaluation. This is **experimental** and enabled under the feature-flag `promql-delayed-name-removal`. #14477
-* [ENHANCEMENT] PromQL: Experimental `sort_by_label` and `sort_by_label_desc` sort by all labels when label is equal. #14655
+* [ENHANCEMENT] PromQL: Experimental `sort_by_label` and `sort_by_label_desc` sort by all labels when label is equal. #14655, #14985
 * [ENHANCEMENT] PromQL: Clarify error message logged when Go runtime panic occurs during query evaluation. #14621
 * [ENHANCEMENT] PromQL: Use Kahan summation for better accuracy in `avg` and `avg_over_time`. #14413
 * [ENHANCEMENT] Tracing: Improve PromQL tracing, including showing the operation performed for aggregates, operators, and calls. #14816

promql/functions.go

@@ -415,22 +415,12 @@ func funcSortDesc(vals []parser.Value, args parser.Expressions, enh *EvalNodeHel
 
 // === sort_by_label(vector parser.ValueTypeVector, label parser.ValueTypeString...) (Vector, Annotations) ===
 func funcSortByLabel(vals []parser.Value, args parser.Expressions, enh *EvalNodeHelper) (Vector, annotations.Annotations) {
-	// First, sort by the full label set. This ensures a consistent ordering in case sorting by the
-	// labels provided as arguments is not conclusive.
+	lbls := stringSliceFromArgs(args[1:])
 	slices.SortFunc(vals[0].(Vector), func(a, b Sample) int {
-		return labels.Compare(a.Metric, b.Metric)
-	})
-
-	labels := stringSliceFromArgs(args[1:])
-	// Next, sort by the labels provided as arguments.
-	slices.SortFunc(vals[0].(Vector), func(a, b Sample) int {
-		// Iterate over each given label.
-		for _, label := range labels {
+		for _, label := range lbls {
 			lv1 := a.Metric.Get(label)
 			lv2 := b.Metric.Get(label)
 
-			// If we encounter multiple samples with the same label values, the sorting which was
-			// performed in the first step will act as a "tie breaker".
 			if lv1 == lv2 {
 				continue
 			}
@@ -442,7 +432,8 @@ func funcSortByLabel(vals []parser.Value, args parser.Expressions, enh *EvalNode
 			return +1
 		}
 
-		return 0
+		// If all labels provided as arguments were equal, sort by the full label set. This ensures a consistent ordering.
+		return labels.Compare(a.Metric, b.Metric)
 	})
 
 	return vals[0].(Vector), nil
@@ -450,22 +441,12 @@ func funcSortByLabel(vals []parser.Value, args parser.Expressions, enh *EvalNode
 
 // === sort_by_label_desc(vector parser.ValueTypeVector, label parser.ValueTypeString...) (Vector, Annotations) ===
 func funcSortByLabelDesc(vals []parser.Value, args parser.Expressions, enh *EvalNodeHelper) (Vector, annotations.Annotations) {
-	// First, sort by the full label set. This ensures a consistent ordering in case sorting by the
-	// labels provided as arguments is not conclusive.
-	slices.SortFunc(vals[0].(Vector), func(a, b Sample) int {
-		return labels.Compare(b.Metric, a.Metric)
-	})
-
-	labels := stringSliceFromArgs(args[1:])
-	// Next, sort by the labels provided as arguments.
+	lbls := stringSliceFromArgs(args[1:])
 	slices.SortFunc(vals[0].(Vector), func(a, b Sample) int {
-		// Iterate over each given label.
-		for _, label := range labels {
+		for _, label := range lbls {
 			lv1 := a.Metric.Get(label)
 			lv2 := b.Metric.Get(label)
 
-			// If we encounter multiple samples with the same label values, the sorting which was
-			// performed in the first step will act as a "tie breaker".
 			if lv1 == lv2 {
 				continue
 			}
@@ -477,7 +458,8 @@ func funcSortByLabelDesc(vals []parser.Value, args parser.Expressions, enh *Eval
 			return -1
 		}
 
-		return 0
+		// If all labels provided as arguments were equal, sort by the full label set. This ensures a consistent ordering.
+		return -labels.Compare(a.Metric, b.Metric)
 	})
 
 	return vals[0].(Vector), nil