to be honest i am not the greatest when it comes to database structure. My understanding is that you should have a primary index on a unique id column and then other index’s on columns that you are going to be searching on. e.g for my postcode table i’d have an index on the postcode field which can actually be the primary unique index as the postcodes should all be different. Basically it means when you do a query it doesn’t have to search every bit of data like in a book you can look at the contents rather than having to read every page. This should give you a better explanation than i can http://dev.mysql.com/doc/refman/5.7/en/mysql-indexes.html
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