from the (pre-edit) link Slower than page-level or table-level locks when used on a large part of the table because you must acquire many more locks use a row level lock if you are only hitting a row or two. If your code hits many or unknown rows, stick with table lock.
[2017] Update: MySQL 5.6 has support for online index updates https://dev.mysql.com/doc/refman/8.0/en/innodb-online-ddl-operations.html#online-ddl-index-syntax-notes In MySQL 5.6 and higher, the table remains available for read and write operations while the index is being created or dropped. The CREATE INDEX or DROP INDEX statement only finishes after all transactions that are accessing the table are completed, so that the … Read more