I use only one primary key on the innodb, table and a clustered index (the default is a clustered index). In my query statement, the where condition is this primary key, but the query column is arbitrary. I found that the extra of the execution plan is not using index,. According to theory, the data field of the leaf node of the clustered index stores the whole row of data, no matter what my query column is, as long as the where condition is the primary key, you can get it from the clustered index.
but the result is that using index, occurs only if the query column is a primary key, otherwise extra is empty
one more question, does innodb support federated clustered indexes?