From 3a908d8e088763e6df7f7a5f74345ada30c9b0bb Mon Sep 17 00:00:00 2001
From: beorn7 <beorn@grafana.com>
Date: Thu, 4 Jul 2024 18:36:32 +0200
Subject: [PATCH] promql: Improve Kahan usage in avg_over_time

The calculation of the mean value in avg_over_time is performed in an
incremental fashion. This introduces additional numerical errors that
even Kahan summation cannot compensate, but at least we can use the
Kahan-corrected mean value when we use the intermediate mean value in
the calculation.

Signed-off-by: beorn7 <beorn@grafana.com>
---
 promql/functions.go | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/promql/functions.go b/promql/functions.go
index dcc2cd759..575f8302d 100644
--- a/promql/functions.go
+++ b/promql/functions.go
@@ -593,7 +593,8 @@ func funcAvgOverTime(vals []parser.Value, args parser.Expressions, enh *EvalNode
 					continue
 				}
 			}
-			mean, c = kahanSumInc(f.F/count-mean/count, mean, c)
+			correctedMean := mean + c
+			mean, c = kahanSumInc(f.F/count-correctedMean/count, mean, c)
 		}
 
 		if math.IsInf(mean, 0) {