Suppose there is a title varchar(250) column and there is an index on it as per below
Alter table mytable add index (title(16));
select id, title from mytable where title=”This is my Title”;
Here query will use index as searching string contains 16 character and our index also take care 16 characters.
select id, title from mytable where title=”This is not my Title but it is yours”;
Here searching string size is greater than index size, so query will go for table scan instead of index usage.