Mysql full table scan results in issues

http://dev.mysql.com/doc/refman/5.0/en/how-to-avoid-table-scan.html
http://dev.mysql.com/doc/refman/5.0/en/explain.html
http://www.percona.com/blog/2014/03/14/tools-and-tips-for-analysis-of-mysqls-slow-query-log/