public override void undo(int txnum)
{
BufferMgr buffMgr = SimpleDB.bufferMgr();
Buffer buff = buffMgr.pin(blk);
buff.setString(offset, val, txnum, -1);
buffMgr.unpin(buff);
}
public void pin(Block blk)
{//让缓冲区绑定一个块，并且在内部字典一直维护这个缓冲区和block的联系
Buffer buff = bufferMgr.pin(blk);
if (!buffers.ContainsKey(blk))
{
buffers.Add(blk, buff);
}
pins.Add(blk);
}
public override void undo(int txnum)
{
BufferMgr buffMgr = SimpleDB.bufferMgr();
//将块绑定到（读入）缓冲区
Buffer buff = buffMgr.pin(blk);
//LSN为负数，生成一条临时日志记录，不保存到磁盘
buff.setInt(offset, val, txnum, -1);
buffMgr.unpin(buff);
}