The key_len column indicates the length of the key that MySQL decided to use.
MySQL's docs mantion this. but i don't figure out how to use this statement to defined a sql is fast or not. the smaller key_len, the faster sql it is(please forgive my english grammarT_T)? and how it works?
expect for your answer.
CodePudding user response：
This parameter makes sense for composite keys only. The server may use either the whole index completely or some prefix of it only. And key_len allows to define what prefix part of composite index is used.