I've some data during my production database already. If I am adding some database indexes on the couple of fields, is the fact that an issue?

Rails 2.3.5, Ruby 1.8.7, sqlite3 database

No, that's no problem. On some implementation of indexes I understand of, the B-Tree really is going to be completely new trees within the database (as pages), so it will not modify the new tree to possess fragmentation. Exactly that building the indexes might take a while for the way much data is incorporated in the DB.

You are able to, but adding indexes cause temporary obstructing tables.