哪个更快，SUB-Query 或 IN-Query？

Question

如果我想 select whether i am following thread 与否。

我有两种方法...但我不知道哪一种在性能和速度方面更好。谁能帮帮我？

方法一

$cui = $_SESSION['user_id'];

$data  = mysqli_query($con,"
SELECT t.*,( select count(follow_id) from follows where object_id = 
t.thread_id AND object_type='thread' AND user_id = $cui) as me_follow FROM threads t
");
  while($row = mysqli_fetch_assoc($data)){
       /*
           $row['me_follow'] = 0 if i aint following
           $row['me_follow'] = 1 if  i am following  
       */
   }

方法二

$cui = $_SESSION['user_id'];

  $data = mysqli_query($con,"SELECT * FROM threads");
  $ids = array();
  while($row = mysqli_fetch_assoc($data)){
       $ids[] = $row['thread_id'];
   }
$ids = join($ids,",");
$data = mysqli_query($con,"SELECT COUNT(*) FROM follows WHERE object_id IN($ids) AND user_id = $cui");

Answer 1

一次往返胜过两次。这是因为将 SQL 发送到服务器、让它解析它、执行它并将结果发回时会产生一些开销。在一次往返中完成所有事情（通常）更好。