What would be better for fast search Lucene.Net or Redis

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http://serverfault.com – I have legacy, for me, Asp MVC solutions, where MySql database using for store users. Unfortunately, it is already more then 3 Million records in database. It is exist unique index for Username field and no any index for email field at user table. By business requirements email should be unique value. Current legacy logic for make this operation is store all email in memory (via Dictionary) and check this dictionary before new insert operation. Somehow email duplication is possible, moreover this Dictionary is using to much memory (this point is not critical). I can't just create unique index (HowTos)