See https://gerrit.wikimedia.org/r/#/c/282807/
Currently the revision tables either have:
PRIMARY KEY (rev_page,rev_id)
UNIQUE KEY (rev_id)
or...
PRIMARY KEY(rev_id)
UNIQUE KEY(rev_page,rev_id)
or...
PRIMARY KEY(rev_id)
KEY(rev_page,rev_id)
Only the last variant makes sense to me. All DBs should be normalized to this.