After reviewing the class's (not very complicated) code, I have some
questions I hope someone can answer:

   - (more general question) Are there many use-cases for using
   DBInputFormat? Do most Hadoop jobs take their input from files or DBs?
   - What happens when the database is updated during mappers' data
   retrieval phase? is there a way to lock the database before the data
   retrieval phase and release it afterwords?
   - Since all mappers open a connection to the same DBS, one cannot use
   hundreds of mapper. Is there a solution to this problem?