可以看到，用id<=1加的锁，只锁住了id<=1的范围，可以成功添加id为2的记录，添加id为0的记录时就会等待锁的释放。
附说明：
MySQL manual里对可重复读里的锁的详细解释：
http://dev.mysql.com/doc/refman/5.0/en/set-transaction.html#isolevel_repeatable-read
For locking reads (SELECT with FOR
UPDATE or LOCK IN SHARE MODE),UPDATE,
and DELETE statements, locking depends on whether the statement uses
a unique index with a unique search condition, or a range-type search condition. For a unique index with a unique search condition, InnoDB locks
only the index record found, not the gap before it. For other search conditions, InnoDB locks
the index range scanned, using gap locks or next-key (gap plus index-record)locks to block insertions by other sessions into the gaps covered by the range.
一致性读和提交读，先看实验，
实验4-4：