by Ken Task.
'shared host' ... might not be able to set 'large prefix' as that's a global, but might be able to 'limp by' using the scripts in /path/to/moodlecode/admin/cli/
php mysql_compressed_rows.php
will show the help/options/switches to command.
php mysql_compressed_rows.php -l
will list problem tables (or should)
php mysql_compressed_rows.php -f
will fix those tables (or should).
Think you'll have to move from 'shared host' to 'dedicated' with DreamHost unless you an talk helpdesk staff to tweak your database.
'spirit of sharing', Ken