Other question, but about the email key:
The cardinality of the primary key is about 40k, and because there're about 40k comments
And the email key, i expected to have less cardinality, because lots of emails are repeated in the table, but has the same as the primary key.
Why is that ? Anything to do that it's varchar utf8_bin ? How can i fix it ?
The primary key is unique so cardinality will always be the number of entries in the table. As for the email, what query are you using to make sure that it it not something in there? Also when the data was inserted are you sure that there was not whitespacing added to either the front or end of the email address which would cause this result? The best way is to find 2 records with the same email address and limit based on that email address only.