How to trigger joint index in MySQL

How to trigger the joint index

1. Create a joint index on the user table username, password

##2. Trigger the joint index

(1) Using all the index keys of the joint index can trigger the joint index

(2) Using all the index keys of the joint index, but connecting with or, the joint index cannot be triggered.

(3) When the first field on the left of the joint index is used alone, the joint index

( 4) When using other fields of the joint index alone, the joint index cannot be triggered

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