mysql row level lock

4.1.3 innodb나 5.1 이상에서 가능

SELECT * FROM t WHERE i = 1 LOCK IN SHARE MODE;

Rows are locked during updates using indexes. For example,
update tab set col1=3 where col2=17;

will lock the entire table unless col2 is indexed (in which case, only rows where col2=17 will be locked).