使用 Straight_Join 的 MySQL 查询成本更低，但执行时间更长

Question

我正在尝试优化以下查询，该查询的执行时间约为 1.5 秒。

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_cstm(id_c,cust_temp_id_c)上创建索引，但没有成功!我也尝试使用 Straight_join，成本降低了，但查询现在需要更多时间来执行(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联接时花费更多时间以及如何优化此查询。

两个表都包含大约 750000 行。

Answer 1

我假设 deleted 位于表 leads 和 leads_cstm 的 cust_temp_id_c 中。

要优化此查询，请尝试添加这两个索引:

ALTER TABLE `leads` ADD INDEX `leads_idx_deleted_id` (`deleted`,`id`);
ALTER TABLE `leads_cstm` ADD INDEX `leads_cstm_idx_c_c` (`cust_temp_id_c`,`id_c`);

然后运行查询:

SELECT
        leads.id 
    FROM
        leads 
    LEFT JOIN
        leads_cstm 
            ON leads.id = leads_cstm.id_c 
    WHERE
        leads.deleted = 0 
        AND leads_cstm.cust_temp_id_c = 'xxxx'