From 93ec767fed82fb947ef890d138016f2872e60ef8 Mon Sep 17 00:00:00 2001
From: Julien Pivotto <291750+roidelapluie@users.noreply.github.com>
Date: Wed, 15 Apr 2026 16:25:58 +0200
Subject: [PATCH] promql: fix smoothed rate returning zero for data only after
 range

For smoothed rate/increase, a result should only be returned
when there is data available to interpolate across the range. If the
range has a single data point only on one side, the result is
meaningless and should be empty.

The "data only before" case was already handled: if the last fetched
sample is at or before rangeStart, extendedRate returns nothing.

Add the symmetric guard for the "data only after" case: if the first
fetched sample is strictly after rangeEnd, return nothing as well.

This mirrors the behaviour described in prometheus/prometheus#18295,
where a smoothed rate that has no data before the range should not
return zero.

Signed-off-by: Julien Pivotto <291750+roidelapluie@users.noreply.github.com>
---
 promql/functions.go                            |  3 +++
 .../promqltest/testdata/extended_vectors.test  | 18 +++++++++++++++++-
 2 files changed, 20 insertions(+), 1 deletion(-)

diff --git a/promql/functions.go b/promql/functions.go
index 546f94df12..2ab7c275e2 100644
--- a/promql/functions.go
+++ b/promql/functions.go
@@ -142,6 +142,9 @@ func extendedRate(vals Matrix, args parser.Expressions, enh *EvalNodeHelper, isC
 	if f[lastSampleIndex].T <= rangeStart {
 		return enh.Out, annos
 	}
+	if smoothed && f[firstSampleIndex].T > rangeEnd {
+		return enh.Out, annos
+	}
 
 	left := pickOrInterpolateLeft(f, firstSampleIndex, rangeStart, smoothed, isCounter)
 	right := pickOrInterpolateRight(f, lastSampleIndex, rangeEnd, smoothed, isCounter)
diff --git a/promql/promqltest/testdata/extended_vectors.test b/promql/promqltest/testdata/extended_vectors.test
index 0bc1140522..06f70913b7 100644
--- a/promql/promqltest/testdata/extended_vectors.test
+++ b/promql/promqltest/testdata/extended_vectors.test
@@ -417,4 +417,20 @@ eval instant at 45s withreset smoothed
     withreset 3
 
 eval instant at 30s notregular smoothed
-    notregular 2
\ No newline at end of file
+    notregular 2
+
+# Smoothed rate returns empty when data is only before or only after the range.
+clear
+load 10s
+    metric _ 5+1x4
+
+eval instant at 5s rate(metric[5s] smoothed)
+
+eval instant at 5s increase(metric[5s] smoothed)
+
+eval instant at 15s increase(metric[10s] smoothed)
+    {} 0.5
+
+eval instant at 60s rate(metric[5s] smoothed)
+
+eval instant at 60s increase(metric[5s] smoothed)