Forcing an index to be used is mostly useful when the optimizer decides to do a table scan even if you know that using an index would be better. (The optimizer could decide to do a table scan even if there is an available index when it believes that most or all rows will match and it can avoid the overhead of using the index).
FORCE INDEX works by only considering the given indexes (like with USE_INDEX) but in addition it tells the optimizer to regard a table scan as something very expensive. However if none of the 'forced' indexes can be used, then a table scan will be used anyway.
FORCE INDEX cannot force an ignored index to be used - it will be treated as if it doesn't exist.
CREATE INDEX Name ON City (Name); EXPLAIN SELECT Name,CountryCode FROM City FORCE INDEX (Name) WHERE name>="A" and CountryCode >="A";
id select_type table type possible_keys key key_len ref rows Extra 1 SIMPLE City range Name Name 35 NULL 4079 Using where
When using index hints (USE, FORCE or IGNORE INDEX), the index name value can also be an unambiguous prefix of an index name.