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create table t(x int, y int);
insert into t values(1,1); --初始值为x = 1
commit;
alter session set isolation_level = read committed;
update t set x = x+1;
select * from t;-- x = 2
alter session set isolation_level = read committed;
update t set x = x+1;--这里读到了transaction 1未提交的数据(x=2),然后执行x = x+1,得到x = 3
select * from t;--x = 3
commit;
select * from t;--x = 3