Created by: yingfeng

I notice that the implementation of SelectedRows is a kind of Key-Value map actually as follows:

Vector<int64_t> rows_; std::unique_ptr<Tensor> value_{nullptr};

together with corresponding typical map operations:

std::vector<std::pair<int64_t, int64_t>> Get(const std::vector<int64_t>& keys,ramework::Tensor* value) const; bool Set(int64_t key, const Tensor& value); However, all Get and Set rely on lookup operations of rows, which is a linear scan actually:

  int64_t Index(int64_t key) const {
    auto it = std::find(rows_.begin(), rows_.end(), key);
    if (it == rows_.end()) {
      return static_cast<int64_t>(-1);
    }
    return static_cast<int64_t>(std::distance(rows_.begin(), it));
  }

Why not choose some kind of lookup table to replace std::vector within SelectedRows?