原SQL语句：(execution: 2 s 28 ms, fetching: 25 ms)

explain
select
    pr.cid,
    case when pr.type = 2 then ac.auto_cancel else ca.auto_cancel end auto_cancel,
    pr.proxy_id proxy_id
from
    crm_scm_cust_proxy_rel pr LEFT JOIN
    crm_company_config_air ca on pr.cid = ca.cid and ca.auto_cancel >0 LEFT JOIN
    pf_air_config ac on ac.face_cid = pr.cid and ac.auto_cancel >0
where
        pr.status = 1 and
    (( pr.type = 1 and ca.auto_cancel >0) or (pr.type = 2 and ac.auto_cancel >0));

改进后：(execution: 156 ms, fetching: 41 ms)

EXPLAIN
select
    pr.cid,
    case when pr.type = 2 then ac.auto_cancel else ca.auto_cancel end auto_cancel,
    pr.proxy_id proxy_id
from
    crm_scm_cust_proxy_rel pr LEFT JOIN
    (select ca.* from crm_company_config_air ca   where ca.auto_cancel >0)ca  on pr.cid = ca.cid  LEFT JOIN
    (select ac.* from pf_air_config   ac where   ac.auto_cancel >0) ac on ac.face_cid = pr.cid
where
        pr.status = 1 and
    (( pr.type = 1 and ca.auto_cancel >0) or (pr.type = 2 and ac.auto_cancel >0));

第二种思路：

explain
select pr.cid,
       case when pr.type = 2 then ac.auto_cancel else ca.auto_cancel end auto_cancel,
       pr.proxy_id                                                       proxy_id
from crm_scm_cust_proxy_rel pr
         LEFT JOIN
     crm_company_config_air ca on pr.cid = ca.cid and ca.auto_cancel > 0
         LEFT JOIN
     pf_air_config ac on ac.face_cid = pr.cid and ac.auto_cancel > 0
where pr.status = 1
  and ((pr.type = 1 and ca.auto_cancel > 0))
union all
select pr.cid,
       case when pr.type = 2 then ac.auto_cancel else ca.auto_cancel end auto_cancel,
       pr.proxy_id                                                       proxy_id
from crm_scm_cust_proxy_rel pr
         LEFT JOIN
     crm_company_config_air ca on pr.cid = ca.cid and ca.auto_cancel > 0
         LEFT JOIN
     pf_air_config ac on ac.face_cid = pr.cid and ac.auto_cancel > 0
where pr.status = 1
  and (pr.type = 2 and ac.auto_cancel > 0);