Why query one of explain result? about yii2-debug HOT 5 CLOSED

$query = 'SELECT * FROM qa_question LEFT JOIN qa_answer ON qa_answer.question_id = qa_question.id';
$result = Yii::$app->getDb()->createCommand('EXPLAIN ' . $query)->queryAll();

var_dump($result);
array (size=2)
  0 => 
    array (size=10)
      'id' => string '1' (length=1)
      'select_type' => string 'SIMPLE' (length=6)
      'table' => string 'qa_question' (length=11)
      'type' => string 'ALL' (length=3)
      'possible_keys' => null
      'key' => null
      'key_len' => null
      'ref' => null
      'rows' => string '649' (length=3)
      'Extra' => null
  1 => 
    array (size=10)
      'id' => string '1' (length=1)
      'select_type' => string 'SIMPLE' (length=6)
      'table' => string 'qa_answer' (length=9)
      'type' => string 'ref' (length=3)
      'possible_keys' => string 'idx_q' (length=5)
      'key' => string 'idx_q' (length=5)
      'key_len' => string '4' (length=1)
      'ref' => string 'lxpgw_0828.qa_question.id' (length=25)
      'rows' => string '1' (length=1)
      'Extra' => string 'Using where' (length=11)