[ENHANCEMENT] PromQL: use Kahan summation for sum() #14074
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This can give a more precise result, by keeping a separate running compensation value to accumulate small errors.
See https://en.wikipedia.org/wiki/Kahan_summation_algorithm
Possible improvement for #14052 (I won't call it a fix).
I'm re-using
floatMean
which is otherwise unused forsum()
; it would be clearer to add a new field but would cost 8 bytes per output value.Benchmarks show a bigger slow-down than I expected.