归并 SST 控制版本堆积与读放大

275void CompactionMergingIterator::Next() {276  assert(Valid());277  // For the heap modifications below to be correct, current_ must be the278  // current top of the heap.279  assert(current_ == CurrentForward());280  // as the current points to the current record. move the iterator forward.281  if (current_->type == HeapItem::ITERATOR) {282    current_->iter.Next();283    if (current_->iter.Valid()) {284      // current is still valid after the Next() call above.  Call285      // replace_top() to restore the heap property.  When the same child286      // iterator yields a sequence of keys, this is cheap.287      assert(current_->iter.status().ok());288      minHeap_.replace_top(current_);289    } else {290      // current stopped being valid, remove it from the heap.291      considerStatus(current_->iter.status());292      minHeap_.pop();293    }