SQL> set plan on;
SQL> select count(*)
CON> from horse
CON> join color on horse.code_color = color.code_color
CON> where horse.name = color.name;

PLAN JOIN (COLOR NATURAL, HORSE INDEX (HORSE_IDX_NAME))

       COUNT
============
          48

SQL> select count(*)
CON> from horse
CON> join color on horse.code_color = color.code_color
CON> where horse.name = color.name
CON> PLAN JOIN (COLOR NATURAL, HORSE INDEX (HORSE_IDX_NAME));
Statement failed, SQLSTATE = 42000
index HORSE_IDX_NAME cannot be used in the specified plan
SQL>