尝试优化 PL/SQL 查询

Question

我正在尝试提高下面查询的速度，但是必填字段无法更改。所以我被困在这里了。请帮我摆脱这个陷阱。一点提示或灵感也很有帮助！！

select cg.province_id,
   (select count(distinct(c.guidance_user_id))
      from case_guidance c
     where c.guidance_status = '2'
       and c.province_id = cg.province_id) as guidance_cnt,
   (select count(distinct(c.guidance_user_id))
      from case_guidance c
     where c.guidance_status = '2'
       and c.guidance_user_type = 'role.type.teacher'
       and c.province_id = cg.province_id) as guidance_teacher_cnt,
   (select count(distinct(c.guidance_user_id))
      from case_guidance c
     where c.guidance_status = '2'
       and c.guidance_user_type = 'role.type.jyy'
       and c.province_id = cg.province_id) as guidance_jyy_cnt,
   (select count(distinct(c.guidance_user_id))
      from case_guidance c
     where c.guidance_status = '2'
       and c.guidance_user_type = 'role.type.expert'
       and c.province_id = cg.province_id) as guidance_expert_cnt,
   (select count(distinct(c.case_id))
      from case_guidance c
     where c.guidance_status = '2'
       and c.province_id = cg.province_id) as guidance_case_cnt
from case_guidance cg
where cg.province_id is not null
group by cg.province_id
order by guidance_cnt desc

Answer 1

将相关子查询替换为CASE以消除所有连接：

select
    province_id,
    count(distinct(case when guidance_status = '2'                                              then guidance_user_id else null end)) guidance_cnt,
    count(distinct(case when guidance_status = '2' and guidance_user_type = 'role.type.teacher' then guidance_user_id else null end)) guidance_teacher_cnt,
    count(distinct(case when guidance_status = '2' and guidance_user_type = 'role.type.jyy'     then guidance_user_id else null end)) guidance_jyy_cnt,
    count(distinct(case when guidance_status = '2' and guidance_user_type = 'role.type.expert'  then guidance_user_id else null end)) guidance_expert_cnt,
    count(distinct(case when guidance_status = '2'                                              then case_id          else null end)) guidance_case_cnt
from case_guidance
group by province_id;
order by guidance_cnt desc

（我故意将代码行留得特别长，以便条件一致。这有助于弄清楚列之间的区别，并且它们都在做几乎完全相同的事情。）

尝试优化 PL/SQL 查询

Try to optimized a PL/SQL query

oracle

sql-optimization