How to find MySQL queries worth optimizing ?

One question I often get is how one can find out queries which should be optimized. By looking at pt-query-digest report it is easy to find slow queries or queries which cause the large portion of the load on the system but how do we know whenever there is any possibility to make this query run better ? The full answer to this question will indeed require complex analyses as there are many possible ways query can be optimized. There is however one extremely helpful metric which you can use – ratio between rows sent and rows analyzed. Lets look at this example: