使用filesort和temp的mysql查询没有任何order by和group by子句

Question

我正在尝试调整以下查询。

SELECT jsondata_c,
       id_c 
FROM accounts 
LEFT JOIN accounts_cstm 
ON accounts.id = accounts_cstm.id_c  
WHERE ispropagated_c = 0 
AND deleted=0 
AND jsondata_c is not null  
ORDER BY accounts.date_modified DESC LIMIT 5

其执行计划如下

*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: accounts_cstm
   partitions: NULL
         type: ALL
possible_keys: PRIMARY,id_c
          key: NULL
      key_len: NULL
          ref: NULL
         rows: 1325982
     filtered: 9.00
        Extra: Using where; Using temporary; Using filesort
*************************** 2. row ***************************
           id: 1
  select_type: SIMPLE
        table: accounts
   partitions: NULL
         type: eq_ref
possible_keys: PRIMARY,idx_accnt_id_del,idx_accnt_assigned_del
          key: PRIMARY
      key_len: 108
          ref: oasyscrm.accounts_cstm.id_c
         rows: 1
     filtered: 50.00
        Extra: Using where
2 rows in set, 1 warning (0.00 sec)

我不确定为什么查询使用filesort和temp而没有任何order by和group_cstm表的group by子句。 请建议调整此查询的方法，因为它需要大约1分钟才能执行。