FORCE INDEX
FORCE INDEX:
It acts like USE INDEX (key_list) but with the addition that a table scan is assumed to be very expensive.A table scan is used only if there is no way to use one of the given indexes to find rows in the table.Force index does not work, if one would want MySQL to prefer full index scan instead of full table scan and file sort.
Example:
EXPLAIN SELECT * FROM tab1 force index(id) ORDER BY id;
