We revisit the problem of finding small $ε$-separation keys introduced by Motwani and Xu (2008). In this problem, the input is $m$-dimensional tuples $x_1,x_2,\ldots,x_n $. The goal is to find a small subset of coordinates that separates at least $(1-ε){n \choose 2}$ pairs of tuples. They provided a fast algorithm that runs on $Θ(m/ε)$ tuples sampled uniformly at random. We show that the sample size can be improved to $Θ(m/\sqrtε)$. Our algorithm also enjoys a faster running time. To obtain this result, we provide upper and lower bounds on the sample size to solve the following decision problem. Given a subset of coordinates $A$, reject if $A$ separates fewer than $(1-ε){n \choose 2}$ pairs, and accept if $A$ separates all pairs. The algorithm must be correct with probability at least $1-δ$ for all $A$. We show that for algorithms based on sampling: - $Θ(m/\sqrtε)$ samples are sufficient and necessary so that $δ\leq e^{-m}$ and - $Ω(\sqrt{\frac{\log m}ε})$ samples are necessary so that $δ$ is a constant. Our analysis is based on a constrained version of the balls-into-bins problem. We believe our analysis may be of independent interest. We also study a related problem that asks for the following sketching algorithm: with given parameters $α,k$ and $ε$, the algorithm takes a subset of coordinates $A$ of size at most $k$ and returns an estimate of the number of unseparated pairs in $A$ up to a $(1\pmε)$ factor if it is at least $α{n \choose 2}$. We show that even for constant $α$ and success probability, such a sketching algorithm must use $Ω(mk \log ε^{-1})$ bits of space; on the other hand, uniform sampling yields a sketch of size $Θ(\frac{mk \log m}{αε^2})$ for this purpose.