refactor(index): 优化索引扫描和内存管理

- 移除 IxScan::next 中的动态内存分配，使用栈分配替代
- 简化 IxScanFinal 构造函数中的代码
- 重构 Page 类，使用静态数组替代动态分配

Author: Lin-jingy

src/index/ix_scan.cpp

@@ -11,22 +11,21 @@ See the Mulan PSL v2 for more details. */
 #include "ix_scan.h"
 
 void IxScan::next() {
-    auto node = new IxNodeHandle(ih_->file_hdr_, now);
-    assert(node->is_leaf_page());
-    assert(iid_.slot_no < node->get_size());
+    IxNodeHandle node(ih_->file_hdr_, now);
+    assert(node.is_leaf_page());
+    assert(iid_.slot_no < node.get_size());
 
     // increment slot no
     iid_.slot_no++;
-    if (iid_.page_no != ih_->file_hdr_->last_leaf_ && iid_.slot_no == node->get_size()) {
+    if (iid_.page_no != ih_->file_hdr_->last_leaf_ && iid_.slot_no == node.get_size()) {
         // go to next leaf
         iid_.slot_no = 0;
-        iid_.page_no = node->get_next_leaf();
+        iid_.page_no = node.get_next_leaf();
         Page* next_page = buffer_pool_manager.fetch_page({ih_->fd_, iid_.page_no});
         next_page->RLatch();
         unlatch();        // 释放当前页面
         now = next_page;  // 更新当前页面为下一个叶节点
     }
-    delete node;  // 释放内存
 }
 
 /**

src/index/ix_scan_final.cpp

@@ -18,7 +18,7 @@ IxScanFinal::IxScanFinal(const IxIndexHandle *ih, const Iid &lower, const Iid &u
     Page *page = buffer_pool_manager.fetch_page({ih_->fd_, iid.page_no});
     page->RLatch();
     while (true) {
-        auto node = IxNodeHandle(ih_->file_hdr_, page);
+        IxNodeHandle node(ih_->file_hdr_, page);
 
         while (iid != upper) {
             rids_.push_back(ih_->get_rid(iid));

src/storage/page.h

@@ -54,9 +54,9 @@ class Page {
     friend class RmFileHandle;
 
    public:
-    Page() { data_ = static_cast<char *>(calloc(PAGE_SIZE, sizeof(char))); }
+    Page() = default;
 
-    ~Page() { delete[] data_; }
+    ~Page() = default;
 
     PageId get_page_id() const { return id_; }
 
@@ -89,7 +89,7 @@ class Page {
     /** The actual data that is stored within a page.
      *  该页面在bufferPool中的偏移地址
      */
-    char *data_;
+    char data_[PAGE_SIZE];
 
     /** 脏页判断 */
     bool is_dirty_ = false;