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User.id type INT
Avatar.foreign_key type VARCHAR
When I use this query:
SELECT *
FROM `user`
LEFT JOIN `avatar` ON (`avatar`.`foreign_key` = `user`.`id` )
Indexes don’t use in this query. Unless I change User.id, Avatar.foreign_key to same type.
How can I make above query use index that I don’t need to change 2 columns type? I want keep VARCHAR and INT on 2 columns.
They need to be the same type–and they should be anyway. If you need one of them to be a VARCHAR, and the other to be an INT, that’s a sign that you’re abusing the VARCHAR column. You should add a new INT column to your avatar table that can have a proper FK relationship to the other.
Why do you need one of these to be a VARCHAR?