Rewriting a slow SQL (sub) query in JOIN

问题

So I've got massive slow SQL query and I've narrowed it down to a slow sub-query, so I want to rewrite it to a JOIN. But I'm stuck... (due to the MAX and GROUP BY)

SELECT *
FROM local.advice AS aa
  LEFT JOIN webdb.account AS oa ON oa.shortname = aa.shortname 
WHERE aa.aa_id = ANY (SELECT MAX(dup.aa_id)
                      FROM local.advice AS dup
                      GROUP BY dup.shortname) 
  AND oa.cat LIKE '111'
ORDER BY aa.ram, aa.cpu DESC
LIMIT 0, 30

回答1:

Here is a different version of your query where the subquery is converted with a join clause

select * from local.advice aa 
JOIN webdb.account oa ON oa.shortname = aa.shortname 
join(
 select max(aa_id) as aa_id,shortname from local.advice 
 group by shortname
)x on x.aa_id = aa.aa_id
where
oa.cat = '111'
order by aa.ram, aa.cpu DESC 
limit 0,30

Also you may need to apply indexes if they are not added already

alter table local.advice add index shortname_idx(shortname);
alter table webdb.account add index cat_shortname_idx(cat,shortname);
alter table local.advice add index ram_idx(ram);
alter table local.advice add index cpu_idx(cpu);

I am assuming aa_id is a primary key so did not add the index

Make sure to take a backup of the tables before applying the indexes

来源：https://stackoverflow.com/questions/30523739/rewriting-a-slow-sql-sub-query-in-join