Examine the mydata table and SELECT statements:
http://www.cndba.cn/hbhe0316/article/22768

You issue:
mysql> begin;
mysql> update mydata set a=0 where b=3;

How many rows are now protected by locks with the default InnoDB configuration?
A. one
B. one row and a next-key lock for supremum
C. one row and a gap-lock
D. five
Answer: D

b列没有索引，所以即使是Innodb，也是加表锁。

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MYSQL