This paper contains two parts. In the first part, we obtain the relations between the classical and paverage Kolmogorov widths for all p, 0 < p < ∞, which is a generalization of the corresponding results of J. Creutzig given in [J. Creutzig, Relations between classical, average and probabilistic Kolmogorov widths, J. Complexity 18 (2002) 287-303]. In the second part, we investigate the best approximation of functions on the weighted Sobolev space BW r 2,µ (B d ) equipped with a centered Gaussian measure by polynomial subspaces in the L q,µ metric for 1 ≤ q < ∞, where L q,µ , 1 ≤ q < ∞, denotes the weighted L q space of functions on the unit ball B d with respect to the weight (1 − |x| 2 ) µ− 1 2 , µ ≥ 0. The asymptotic orders of the average error estimations are obtained. We find a striking fact that, in the average case setting, the polynomial subspaces are the asymptotically optimal linear subspaces in the L q,µ metric only for 1 ≤ q < 2 + 1/µ, which means that 2 + 1/µ is the critical value and is independent of dimension d.