[tor-relays] GeoIP file

How does the tor daemon read the GeoIP database file? Does it read the whole file once when starting up, or every time it needs to resolve an IP, or something in between (say, it builds an index in memory on startup and then seeks to locations in the file when looking up)?

I am asking because I want to know if I need to restart, SIGHUP or in some other way kick the daemon after I install a new GeoIP file into place.