feat: v0.2.4 异步Compaction + 槽位压缩 + EXPLAIN + 慢查询日志

src/engine/aria/page/slot.ts

@@ -128,6 +128,52 @@ export function freeSlot(buf: ArrayBuffer, slotIndex: number): void {
   setSlotEntry(buf, slotIndex, { offset: 0, length: 0 });
 }
 
+/**
+ * 压缩页面槽位：移除已删除 slot，整理碎片空间。
+ * 将有效数据紧凑排列，释放空洞。
+ */
+export function compactSlots(buf: ArrayBuffer): number {
+  const view = new DataView(buf);
+  const slotCount = view.getUint16(9, false);
+  if (slotCount === 0) return 0;
+
+  // 收集有效 slot（offset>0 的）
+  const validSlots: { index: number; offset: number; length: number; data: Uint8Array }[] = [];
+  for (let i = 0; i < slotCount; i++) {
+    const entry = getSlotEntry(buf, i);
+    if (entry.offset > 0 && entry.length > 0) {
+      const data = new Uint8Array(buf, entry.offset, entry.length);
+      validSlots.push({ index: i, offset: entry.offset, length: entry.length, data: new Uint8Array(data) });
+    }
+  }
+
+  if (validSlots.length === slotCount) return 0; // 无碎片
+
+  // 从页面底部重新紧凑排列
+  let dataEnd = PAGE_SIZE;
+  const newSlots: { offset: number; length: number }[] = [];
+
+  for (let i = validSlots.length - 1; i >= 0; i--) {
+    const s = validSlots[i];
+    dataEnd -= s.length;
+    new Uint8Array(buf).set(s.data, dataEnd);
+    newSlots.unshift({ offset: dataEnd, length: s.length });
+  }
+
+  // 重写 slot directory
+  view.setUint16(9, validSlots.length, false); // slotCount
+  view.setUint16(7, dataEnd, false); // freeEnd
+  for (let i = 0; i < validSlots.length; i++) {
+    setSlotEntry(buf, i, newSlots[i]);
+  }
+  // 清除剩余 slot 条目
+  for (let i = validSlots.length; i < slotCount; i++) {
+    setSlotEntry(buf, i, { offset: 0, length: 0 });
+  }
+
+  return slotCount - validSlots.length; // 回收的 slot 数量
+}
+
 /**
  * 读取指定 slot 的行数据。
  */