在read committed隔离级别下,有两个session对数据库InnoDB表tab操作进行的时间轴,假设最初表中有且只有一行a=2的记录,并且都没有其他人在操作数据库,下面描述中正确的是: session A session B time1 begin ; time2 update tab set a = 1 where a = 2 ; begin; time3 select * from tab where a = 1 ; time4 commit ; time5 select * from tab where a = 1 ; time6 begin ; time7 update tab set a = 2 where a = 1 ; select * from tab where a = 1 ; time9 commit ; commit ;