<tl;dr> Where an index is desired on a mysql
column of base36 encoded sha1
hashes, I recommend ADD INDEX (sha1column(10)). Shorter indexes will be
sufficient in many cases, but this is still provides a >2/3 space savings
while covering a huge (2^51.43) space.
Isn't it better to store BIGINT containing part of (binary) sha1 and
use index on numeric column?