type shows the access type, which is a more important indicator. The result values from good to bad are:
system > const > eq_ref > ref > fulltext > ref_or_null > index_merge > unique_subquery > index_subquery > range > index > ALL
Generally speaking, it is necessary to ensure that the query reaches at least the range level, preferably ref.
Reference link: http://database.51cto.com/art/201108/284783.htm