I investigated a bit more... the terminal 'annotate' uses a query like this:
<verbatim>
  mid = db_int(0, "SELECT mid FROM mlink WHERE fid=%d AND fnid=%d", fid, fnid);
</verbatim>

that does not give a SINGLE result. Different checkins may have the very same file artifact, and it needs further code to decide what checkin is of interest.

Then, it needs some code to choose what is of the interest of the requester.