MQE: Ensure we never skip buckets for histogram_stddev and histogram_stdvar

CHANGELOG.md

@@ -33,6 +33,7 @@
 * [BUGFIX] MQE: Fix an issue where series were joined in binary operations using the wrong labels when `group_left()`/`group_right()` were used in combination with `ignoring()`. This bug manifested as valid queries returning an error `grouping labels must ensure unique matches`. #16387
 * [BUGFIX] MQE: Fix queries and rules containing a subquery whose range is shorter than its step (e.g. `foo[10m:3d]`) failing with `last bucket must not be before first bucket`. #16442
 * [BUGFIX] MQE: Fix `sum_over_time()` over native histograms losing precision when experimental range vector splitting is enabled. The Kahan compensation of each split range was discarded instead of being carried over to the other split ranges. #16455
+* [BUGFIX] MQE: Never skip decoding histogram buckets for `histogram_stddev` and `histogram_stdvar`, as this could return wrong results when these functions are nested in other functions that skip decoding histogram buckets like `histogram_sum`, `histogram_count` or `histogram_avg`. #16486
 * [BUGFIX] Upgrade Go to 1.26 latest with fixes for [CVE-2026-33818](https://pkg.go.dev/vuln/GO-2026-5972), [CVE-2026-39821](https://pkg.go.dev/vuln/GO-2026-5026), [CVE-2026-46600](https://pkg.go.dev/vuln/GO-2026-5942), [CVE-2026-56853](https://pkg.go.dev/vuln/GO-2026-6089), [CVE-2026-56858](https://pkg.go.dev/vuln/GO-2026-6091), [CVE-2026-56859](https://pkg.go.dev/vuln/GO-2026-6088), [CVE-2026-56860](https://pkg.go.dev/vuln/GO-2026-6218), and [CVE-2026-56862](https://pkg.go.dev/vuln/GO-2026-6090). #16408 #16430
 * [BUGFIX] Store-gateway: Drain the chunks range reader before closing it so HTTP object storage connections can be reused. #16338
 * [BUGFIX] Block-builder-scheduler: Fail startup instead of silently switching to normal operation without assigning any jobs when probing the initial consumption offsets fails. #16028

pkg/streamingpromql/optimize/plan/skip_histogram_decoding.go

@@ -56,7 +56,7 @@ func (s *SkipHistogramDecodingOptimizationPass) applyToNode(node planning.Node,
 		switch f.Function {
 		case functions.FUNCTION_HISTOGRAM_COUNT, functions.FUNCTION_HISTOGRAM_SUM, functions.FUNCTION_HISTOGRAM_AVG:
 			skipHistogramBuckets = true
-		case functions.FUNCTION_HISTOGRAM_FRACTION, functions.FUNCTION_HISTOGRAM_QUANTILE, functions.FUNCTION_HISTOGRAM_QUANTILES:
+		case functions.FUNCTION_HISTOGRAM_FRACTION, functions.FUNCTION_HISTOGRAM_QUANTILE, functions.FUNCTION_HISTOGRAM_QUANTILES, functions.FUNCTION_HISTOGRAM_STDDEV, functions.FUNCTION_HISTOGRAM_STDVAR:
 			skipHistogramBuckets = false
 		default:
 			// Nothing to do.

pkg/streamingpromql/optimize/plan/skip_histogram_decoding_test.go

@@ -70,6 +70,22 @@ func TestSkipHistogramDecodingOptimizationPass(t *testing.T) {
 						- param 2: VectorSelector: {__name__="some_metric"}
 			`,
 		},
+		"single vector selector with histogram_stddev": {
+			expr: `histogram_stddev(some_metric)`,
+			expectedPlan: `
+				- DeduplicateAndMerge
+					- FunctionCall: histogram_stddev(...)
+						- VectorSelector: {__name__="some_metric"}
+			`,
+		},
+		"single vector selector with histogram_stdvar": {
+			expr: `histogram_stdvar(some_metric)`,
+			expectedPlan: `
+				- DeduplicateAndMerge
+					- FunctionCall: histogram_stdvar(...)
+						- VectorSelector: {__name__="some_metric"}
+			`,
+		},
 		"vector selector eligible for skipping decoding in binary expression": {
 			expr: `2 * histogram_sum(some_metric)`,
 			expectedPlan: `
@@ -126,6 +142,30 @@ func TestSkipHistogramDecodingOptimizationPass(t *testing.T) {
 									- param 1: VectorSelector: {__name__="some_other_metric"}
 			`,
 		},
+		"inner vector selector not eligible for skipping decoding due to nesting inside histogram_stddev": {
+			expr: `histogram_sum(some_metric * histogram_stddev(some_other_metric))`,
+			expectedPlan: `
+				- DeduplicateAndMerge
+					- FunctionCall: histogram_sum(...)
+						- BinaryExpression: LHS * RHS
+							- LHS: VectorSelector: {__name__="some_metric"}, skip histogram buckets
+							- RHS: DeduplicateAndMerge
+								- FunctionCall: histogram_stddev(...)
+									- VectorSelector: {__name__="some_other_metric"}
+			`,
+		},
+		"inner vector selector not eligible for skipping decoding due to nesting inside histogram_stdvar": {
+			expr: `histogram_sum(some_metric * histogram_stdvar(some_other_metric))`,
+			expectedPlan: `
+				- DeduplicateAndMerge
+					- FunctionCall: histogram_sum(...)
+						- BinaryExpression: LHS * RHS
+							- LHS: VectorSelector: {__name__="some_metric"}, skip histogram buckets
+							- RHS: DeduplicateAndMerge
+								- FunctionCall: histogram_stdvar(...)
+									- VectorSelector: {__name__="some_other_metric"}
+			`,
+		},
 		"both vector selectors eligible for skipping decoding despite nesting": {
 			expr: `histogram_sum(some_metric * histogram_count(some_other_metric))`,
 			expectedPlan: `

pkg/streamingpromql/testdata/ours/native_histograms.test

@@ -34,6 +34,16 @@ eval range from 0 to 5m step 1m histogram_stdvar(single_histogram)
 eval range from 0 to 5m step 1m histogram_sum(single_histogram)
   {} 5 5 5 5 5 20
 
+# histogram_stddev/histogram_stdvar operate on the buckets, so their input's buckets must never be skipped,
+# even when nested under histogram_sum/histogram_count/histogram_avg (which would otherwise enable skipping via
+# the "skip decoding histogram buckets" optimization). If the buckets were skipped, histogram_stddev/histogram_stdvar
+# would see an empty bucket set and return 0, making the surrounding histogram_sum 0 as well.
+eval instant at 5m histogram_sum(single_histogram * histogram_stddev(single_histogram))
+  {} 59.72564428477802
+
+eval instant at 5m histogram_sum(single_histogram * histogram_stdvar(single_histogram))
+  {} 178.35762926159188
+
 clear
 
 # Test metric with mixed floats and histograms