Merge pull request #13852 from prometheus/fix-hist-std-dev-var-negative

Fix hist std dev var negative

promql/engine_test.go

@@ -3514,7 +3514,39 @@ func TestNativeHistogram_HistogramStdDevVar(t *testing.T) {
 				},
 				NegativeBuckets: []int64{1, 0},
 			},
-			stdVar: 1544.8582535368798, // actual variance: 1738.4082
+			stdVar: 1844.4651144196398, // actual variance: 1738.4082
+		},
+		{
+			name: "-100000, -10000, -1000, -888, -888, -100, -50, -9, -8, -3",
+			h: &histogram.Histogram{
+				Count:     10,
+				ZeroCount: 0,
+				Sum:       -112946,
+				Schema:    0,
+				NegativeSpans: []histogram.Span{
+					{Offset: 2, Length: 3},
+					{Offset: 1, Length: 2},
+					{Offset: 2, Length: 1},
+					{Offset: 3, Length: 1},
+					{Offset: 2, Length: 1},
+				},
+				NegativeBuckets: []int64{1, 0, 0, 0, 0, 2, -2, 0},
+			},
+			stdVar: 759352122.1939945, // actual variance: 882690990
+		},
+		{
+			name: "-10 x10",
+			h: &histogram.Histogram{
+				Count:     10,
+				ZeroCount: 0,
+				Sum:       -100,
+				Schema:    0,
+				NegativeSpans: []histogram.Span{
+					{Offset: 4, Length: 1},
+				},
+				NegativeBuckets: []int64{10},
+			},
+			stdVar: 1.725830020304794, // actual variance: 0
 		},
 		{
 			name: "-50, -8, 0, 3, 8, 9, 100, NaN",

promql/functions.go

@@ -1111,11 +1111,17 @@ func funcHistogramStdDev(vals []parser.Value, args parser.Expressions, enh *Eval
 		it := sample.H.AllBucketIterator()
 		for it.Next() {
 			bucket := it.At()
+			if bucket.Count == 0 {
+				continue
+			}
 			var val float64
 			if bucket.Lower <= 0 && 0 <= bucket.Upper {
 				val = 0
 			} else {
 				val = math.Sqrt(bucket.Upper * bucket.Lower)
+				if bucket.Upper < 0 {
+					val = -val
+				}
 			}
 			delta := val - mean
 			variance, cVariance = kahanSumInc(bucket.Count*delta*delta, variance, cVariance)
@@ -1144,11 +1150,17 @@ func funcHistogramStdVar(vals []parser.Value, args parser.Expressions, enh *Eval
 		it := sample.H.AllBucketIterator()
 		for it.Next() {
 			bucket := it.At()
+			if bucket.Count == 0 {
+				continue
+			}
 			var val float64
 			if bucket.Lower <= 0 && 0 <= bucket.Upper {
 				val = 0
 			} else {
 				val = math.Sqrt(bucket.Upper * bucket.Lower)
+				if bucket.Upper < 0 {
+					val = -val
+				}
 			}
 			delta := val - mean
 			variance, cVariance = kahanSumInc(bucket.Count*delta*delta, variance, cVariance)