title: mysql index: possible_keys is null but key isn't null
body: <p>Today, there is a problem using a composite index, such as (<code>name, age, DESC</code>), and now go to query <code>"select * from table ThisTableName where age&gt;15 and desc='test' "</code>. In this case, the B-Tree index is not in line with the leftmost matching principle, the index possible_keys for null can be understood, but key has shown that the composite index. Why this is the case, possible_keys is representative of the index can be used, if this value is null, key value in principle only for null.</p>