Class HeapFileIterator

src/java/simpledb/storage/HeapFile.java:185–269

这个迭代器的作用是用来遍历所有的tuple，但是不要将所有tuple一次性放入内存，而是一页一页的读和遍历

     * 这个迭代器的作用是用来遍历所有的tuple，但是不要将所有tuple一次性放入内存，而是一页一页的读和遍历
     */
    public class HeapFileIterator implements DbFileIterator{
        TransactionId tid;
        Permissions permissions;
        BufferPool bufferPool =Database.getBufferPool();
        Iterator<Tuple> iterator;  //这个iterator是每一页的迭代器
        int num = 0;

        public HeapFileIterator(TransactionId tid,Permissions permissions){
            this.tid = tid;
            this.permissions = permissions;
        }

        /**
         * 开始进行遍历，默认从第一页开始
         * @throws DbException
         * @throws TransactionAbortedException
         */
        @Override
        public void open() throws DbException, TransactionAbortedException {
            num = 0;
            HeapPageId heapPageId = new HeapPageId(getId(), num);
            HeapPage page = (HeapPage)this.bufferPool.getPage(tid, heapPageId, permissions);
            if(page==null){
                throw  new DbException("page null");
            }else{
                iterator = page.iterator();
            }
        }

        /**
         * 获取下一有数据的页
         * @return
         * @throws DbException
         * @throws TransactionAbortedException
         */
        public boolean nextPage() throws DbException, TransactionAbortedException {
            while(true){
                num++;
                if(num>=numPages()){
                    return false;
                }
                HeapPageId heapPageId = new HeapPageId(getId(), num);
                HeapPage page = (HeapPage)bufferPool.getPage(tid,heapPageId,permissions);
                if(page==null){
                    continue;
                }
                iterator = page.iterator();
                if(iterator.hasNext()){
                    return true;
                }
            }
        }



        @Override
        public boolean hasNext() throws DbException, TransactionAbortedException {
            if(iterator==null){