oracle query scan full table and walk index

One is select CLS_DATE where CLS_DATE =X goes to the index, the other is select CLS_DATE, b, c where CLS_DATE =x goes full table scan to create an index on the field CLS_DATE why is this
No, one is a queryThe field is an index field. One is that there are other fields in addition to the query index field. The former uses the index, and the latter uses the whole table.