【事务】可重复读

【实验】

session1
BEGIN TRANSACTION ISOLATION LEVEL REPEATABLE READ;

select count(1) from t1;

session2

insert into t1(id) values(1);

结果：session1读不到session2已经提交的数据

【原理】

tuple.xmin>=snapshot.xmax，就不可见

HeapTupleSatisfiesMVCC

  XidInMVCCSnapshot(HeapTupleHeaderGetRawXmin(tuple), snapshot)==true   

      if (TransactionIdFollowsOrEquals(xid, snapshot->xmax))
        #两个相同，返回true

  #return false