我试图优化下面的查询，这是大约1.5秒执行。

SELECT  id
    FROM  leads
    left join  leads_cstm  ON leads.id = leads_cstm.id_c
    WHERE  deleted=0
      and  cust_temp_id_c = 'xxxx';

***************************1. row***************************

           id: 1
  select_type: SIMPLE
        table: leads_cstm
   partitions: NULL
         type: ALL
possible_keys: PRIMARY
          key: NULL
      key_len: NULL
          ref: NULL
         rows: 696334
     filtered: 10.00
        Extra: Using where

***************************2. row***************************

           id: 1
  select_type: SIMPLE
        table: leads
   partitions: NULL
         type: eq_ref
possible_keys: PRIMARY,idx_del_user,idx_leads_id_del
          key: PRIMARY
      key_len: 108
          ref: crmsuite.leads_cstm.id_c
         rows: 1
     filtered: 50.00
        Extra: Using where
2 rows in set, 1 warning (0.00 sec)

我尝试在leads\u cstm（id\u c，cust\u temp\u id\u c）上创建索引，但没有成功！！我也尝试过使用直接的连接，降低了成本，但是查询现在需要更多的时间来执行（3秒）。

mysql>  EXPLAIN     SELECT  id
    FROM  leads
    STRAIGHT_JOIN  leads_cstm  ON leads.id = leads_cstm.id_c
    WHERE  deleted=0
      and  cust_temp_id_c = 'xxxxx';

***************************1. row***************************

           id: 1
  select_type: SIMPLE
        table: leads
   partitions: NULL
         type: ref
possible_keys: PRIMARY,idx_del_user,idx_leads_id_del
          key: idx_del_user
      key_len: 2
          ref: const
         rows: 375820
     filtered: 100.00
        Extra: Using index

***************************2. row***************************

           id: 1
  select_type: SIMPLE
        table: leads_cstm
   partitions: NULL
         type: eq_ref
possible_keys: PRIMARY
          key: PRIMARY
      key_len: 108
          ref: crmsuite.leads.id
         rows: 1
     filtered: 10.00
        Extra: Using where
2 rows in set, 1 warning (0.00 sec)

请说明为什么使用直接连接需要更多时间，以及如何优化此查询。
两个表都包含大约750000行。