feature: [or performance] optimize the performance of the or operation by reducing the marshaling call from O(m*n) to O(m+n) when large volumes of series are involved

app/vmselect/promql/binary_op.go

@@ -566,15 +566,24 @@ func fillLeftNaNsWithRightValuesOrMerge(tssLeft, tssRight []*timeseries) {
 		return
 	}
 
-	nameLeft, nameRight := bbPool.Get(), bbPool.Get()
+	// to avoid marshaling the same metric name multiple times, use a slice to store the results and an index to access them.
+	// todo: this may allocate a lot of []byte and could be further optimized.
+	nameRightSlice := make([][]byte, len(tssRight))
+	getRightName := func(idx int) []byte {
+		if nameRightSlice[idx] == nil {
+			nameRightSlice[idx] = marshalMetricNameSorted(nameRightSlice[idx], &tssRight[idx].MetricName)
+		}
+		return nameRightSlice[idx]
+	}
+
+	nameLeft := bbPool.Get()
 	for _, tsLeft := range tssLeft {
 		valuesLeft := tsLeft.Values
 		nameLeft.B = marshalMetricNameSorted(nameLeft.B[:0], &tsLeft.MetricName)
 		for i, v := range valuesLeft {
 			leftIsNaN := math.IsNaN(v)
-			for _, tsRight := range tssRight {
-				nameRight.B = marshalMetricNameSorted(nameRight.B[:0], &tsRight.MetricName)
-				canBeMerged := bytes.Equal(nameLeft.B, nameRight.B)
+			for rIdx, tsRight := range tssRight {
+				canBeMerged := bytes.Equal(nameLeft.B, getRightName(rIdx))
 				valueRight := tsRight.Values[i]
 				if leftIsNaN && canBeMerged {
 					// fill NaNs with valueRight if labels match
@@ -589,7 +598,6 @@ func fillLeftNaNsWithRightValuesOrMerge(tssLeft, tssRight []*timeseries) {
 		}
 	}
 	bbPool.Put(nameLeft)
-	bbPool.Put(nameRight)
 }
 
 func binaryOpIfnot(bfa *binaryOpFuncArg) ([]*timeseries, error) {

app/vmselect/promql/binary_op_timing_test.go

@@ -92,6 +92,22 @@ func BenchmarkBinaryOpOr(b *testing.B) {
 		}
 		benchmarkBinaryOpOr(b, bfa)
 	})
+
+	b.Run("tss:1000 or tss:40000: new", func(b *testing.B) {
+		left, right := make([]*timeseries, 1000), make([]*timeseries, 40000)
+		for i := range left {
+			left[i] = ts(fmt.Sprintf(`a{foo="%d"}`, i))
+		}
+		for i := range right {
+			right[i] = ts(fmt.Sprintf(`b{foo="%d"}`, i))
+		}
+		bfa := &binaryOpFuncArg{
+			be:    mustParseMetricsQL("a or b"),
+			left:  left,
+			right: right,
+		}
+		benchmarkBinaryOpOr(b, bfa)
+	})
 }
 
 func benchmarkBinaryOpOr(b *testing.B, bfa *binaryOpFuncArg) {

docs/victoriametrics/changelog/CHANGELOG.md

@@ -26,6 +26,8 @@ See also [LTS releases](https://docs.victoriametrics.com/victoriametrics/lts-rel
 
 ## tip
 
+* FEATURE: [vmsingle](https://docs.victoriametrics.com/victoriametrics/single-server-victoriametrics/) and `vmselect` in [VictoriaMetrics cluster](https://docs.victoriametrics.com/victoriametrics/cluster-victoriametrics/): optimize the performance of the `or` operation by reducing the marshaling call from `O(m*n)` to `O(m+n)` when large volumes of series are involved. See [#10374](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/10374).
+
 ## [v1.135.0](https://github.com/VictoriaMetrics/VictoriaMetrics/releases/tag/v1.135.0)
 
 Released at 2026-01-30