Milter-greylist

Questions about greylist.db file:

1. Why in a file greylist.db the text  list, instead of dbm-hash is used?
At me in it more than two hundred thousand records. At such quantity of 
dbm-structure give appreciable increase in productivity.

2. Why at registration of the incoming letters in greylist.db such 
convenient parameter, how Message-ID is not used?

Ogogon.