MySQL 5.7 Reference Manual Including MySQL NDB Cluster 7.5 and NDB Cluster 7.6
These views display indexes for which there are no events, which indicates that they are not being used. By default, rows are sorted by schema and table.
This view is most useful when the server has been up and processing long enough that its workload is representative. Otherwise, presence of an index in this view may not be meaningful.
          The schema_unused_indexes view
          has these columns:
        
              object_schema
            
The schema name.
              object_name
            
The table name.
              index_name
            
The unused index name.