This is actually very opaque problem to somebody who doesn't use Homebrew. Why
did `brew' want to install mysql package? Maybe it's percona-toolkit's
maintainer who fscked up the job and put a non-alternative dependency on
"mysql"?
Well, being too smart about binary packages after supporting source packages
also could give similar results. Gentoo's emerge used heuristics and heavy
magic when used with tbz2s, and that failed every now and then.
From package managers I like most how aptitude allows to browse resolutions
and allows to choose one and modify it, but this is very far from being
trivial to implement.
This issue, just closed without a solution is evidence of the terrible dependency management system homebrew is built on.