bool Item_func_in::resolve_type(THD *thd){
  ......
  // 判斷 in 條件字段本身是不是 json 類型
  bool compare_as_json = (args[0]->data_type() == MYSQL_TYPE_JSON);
  ......
  bool values_are_const = true;
  Item **arg_end = args + arg_count;
  for (Item **arg = args + 1; arg != arg_end; arg++) {
    // 判斷 in 條件括號(hào)中的值是不是 json 類型
    compare_as_json |= (arg[0]->data_type() == MYSQL_TYPE_JSON);
    // 判斷 in 條件括號(hào)中的值是不是常量
    if (!(*arg)->const_item()) {
      // in 條件括號(hào)中的值，只要有一個(gè)不是常量，就記錄下來
      values_are_const = false;
      // @todo - rewrite as has_subquery() ???
      if ((*arg)->real_item()->type() == Item::SUBSELECT_ITEM)
        dep_subq_in_list = true;
      // 然后結(jié)束循環(huán)
      break;
    }
  }
  ......
}

bool Item_func_in::resolve_type(THD *thd){
  ......
  uint type_cnt = 0;
  for (uint i = 0; i <= (uint)DECIMAL_RESULT; i++) {
    if (found_types & (1U << i)) {
      (type_cnt)++;
      cmp_type = (Item_result)i;
    }
  }
  ......
}

bool Item_func_in::resolve_type(THD *thd){
  ......
  /*
    First conditions for bisection to be possible:
     1. All types are similar, and
     2. All expressions in <in value list> are const
     3. No JSON is compared (in such case universal JSON comparator is used)
  */
  bool bisection_possible = type_cnt == 1 &&     // 1
                            values_are_const &&  // 2
                            !compare_as_json;    // 3
  ......
}

// items 就是用于存放 in 條件括號(hào)中所有值的數(shù)組
bool in_vector::fill(Item **items, uint item_count){
  used_count = 0;
  for (uint i = 0; i < item_count; i++) {
    // in 條件括號(hào)中的值加入數(shù)組
    set(used_count, items[i]);
    /*
      We don't put NULL values in array, to avoid erroneous matches in
      bisection.
    */
    // include this cell in the array.
    if (!items[i]->null_value) used_count++;
  }
  assert(used_count <= count);
  // 對(duì)數(shù)組元素進(jìn)行排序
  resize_and_sort();

  // True = at least one null value found.
  return used_count < item_count;
}

bool in_longlong::find_item(Item *item){
  if (used_count == 0) return false;
  packed_longlong result;
  // 從記錄中讀取 in 字段的值到 result
  val_item(item, &result);
  // 讀取出來的記錄中，in 字段值為 null 就不用二分法查找了
  if (item->null_value) return false;
  // 對(duì)于非 null 值進(jìn)行二分法查找
  return std::binary_search(base.begin(), base.end(), result, Cmp_longlong());
}

longlong Item_func_in::val_int(){
  ......
  for (uint i = 1; i < arg_count; i++) {
    // in 條件括號(hào)中當(dāng)前循環(huán)的值是 null，跳過
    if (args[i]->real_item()->type() == NULL_ITEM) {
      have_null = true;
      continue;
    }
    
    // 獲取 in 條件括號(hào)中的值，用什么類型進(jìn)行比較，記為 cmp_type
    Item_result cmp_type =
        item_cmp_type(left_result_type, args[i]->result_type());
    in_item = cmp_items[(uint)cmp_type];
    assert(in_item);
    if (!(value_added_map & (1U << (uint)cmp_type))) {
      // 把讀取出來的記錄的 in 字段值轉(zhuǎn)換為 cmp_type 類型
      in_item->store_value(args[0]);
      value_added_map |= 1U << (uint)cmp_type;
      if (current_thd->is_error()) return error_int();
    }
    // 比較讀取出來的記錄的 in 字段值和當(dāng)前循環(huán)的值是否相等
    const int rc = in_item->cmp(args[i]);
    // rc 就是比較結(jié)果，false 表示相等
    if (rc == false) return (longlong)(!negated);
    have_null |= (rc == UNKNOWN);
    if (current_thd->is_error()) return error_int();
  }
  ......
}