I have a little statistics question about the variance to mean (V/M)
value reported by mk-query digest.
let's consider this output :
# Attribute pct total min max avg 95% stddev
# ============ === ======= ======= ======= ======= ======= =======
# Count 12 160000
# Exec time 11 152s 0 1s 948us 972us 8ms
(i have added the v/m column from the profile output).
From my understanding :
* variance V(X) is the mean of the squared distances to the mean
* std dev is the squared root of the variance
* variance to mean = the ratio of the variance (σ) squared to the mean
(µ) : σ^2/ μ
* coefficient of dispersion = variance to mean : σ / µ
For the sample above, we have :
stddev = 0.008 = sqrt(variance)
variance = 0.008^2 = 64µs = 0.000064s
coefficient of dispersion = 0.000064 / 0.000948 ~= 0.07
variance to mean ratio = 0.000064^2 / 0.000948 ~= 0.000004
Based on the sample above, i think mk-query-digest reports the
coefficient of dispersion, not the variance to mean.