优化SQL语句

app/Http/Controllers/Api/MessageController.php

@@ -16,16 +16,13 @@ class MessageController extends Controller
 {
     public function index()
 	{
-		$list = Message::where('agent_id', $this->agent_id)
+		$list = Message::query()->where('agent_id', $this->agent_id)
 			->whereIn('user_id', [$this->user_id, 0]) //0是所有人都接收的系统消息
-			->select('id', 'user_id', 'title', 'created_at')
+			->select('id', 'title', 'created_at')
+			->selectRaw('IF(( SELECT 1 FROM `message_reads` AS `mr` WHERE mr.id = messages.id AND mr.user_id = messages.user_id ), 1, 0) AS `is_read`')
 			->orderBy('id', 'DESC')
 			->simplePaginate(15);
-		if (!empty($list->items())) {
-			foreach ($list->items() as &$v) {
-				$v->is_read = (int)MessageRead::where(['id' => $v->id, 'user_id' => $this->user_id])->exists();
-			}
-		}
+
 		return $this->success($list);
 	}