In the article about the role of a primary key, I mentioned that a secondary index in an InnoDB table consists not only of the values of its member columns, but also values of the table’s primary key are concatenated to the index. I.e. the primary key contents is part of every other index.
Assuming the following table structure:
CREATE TABLE `bets` ( `id` int(10) unsigned NOT NULL, `user_id` int(10) unsigned NOT NULL, `game_id` int(10) unsigned NOT NULL, ... PRIMARY KEY (`id`), KEY `user_id` (`user_id`) ) ENGINE=InnoDB
Here is the visualization:
If MySQL could use in queries these implicitly[Read more...]