MYSQL带有子查询的中等查询优化

Question

I have this query that returns the following:

• Region name
• Region ID
• RANK (Position) of this Region, based on the total number of votes
• Nb of distinct users from this region
• Nb of distinct photos from this region

The query take about ~7.5s to complete... I'd like some advice to optimize my query.

            select
            WrappedQuery.*,
            regions.name as region_name,
            regions.id as region_id,
            count(distinct users.id) as nb_users,
            count(distinct photos.id) as nb_photos
            from (
                select
                  @rownum := @rownum +1 as rank,
                  prequery.region_id,
                  prequery.VoteCount
                from
                  ( select @rownum := 0 ) sqlvars,
                  ( select region_id, count(id) VoteCount
                      from votes
                      where theme_id = '{$currentTheme}'
                      group by region_id
                      order by count(id) desc ) prequery
              ) WrappedQuery, regions, users, photos
              WHERE regions.id = WrappedQuery.region_id
              AND users.region_id = WrappedQuery.region_id
              AND photos.region_id = WrappedQuery.region_id
              GROUP BY WrappedQuery.region_id
              ORDER BY WrappedQuery.rank ASC
              LIMIT 0, 1

Thanks a lot in advance.

Answer 1

Your query has way too much overhead for what you want to achieve. I've rewritten it for you...

select 
/*you don't need that
@rownum := @rownum +1 as rank, 
*/
regions.name as region_name,
regions.id as region_id,
count(distinct users.id) as nb_users,
count(distinct photos.id) as nb_photos,
count(votes.id) as VoteCount
from votes
INNER JOIN regions ON votes.region_id = regions.id
INNER JOIN users ON users.region_id = regions.id
INNER JOIN photos ON photos.region_id = regions.id
/*you don't need that
, ( select @rownum := 0 ) sqlvars
*/
where theme_id = '{$currentTheme}'
group by regions.id
order by VoteCount DESC
LIMIT 1

I commented out the part with the rank, since you just want 1 row anyway.

If it still is too slow, you have to post the result of EXPLAIN SELECT .../*the query from above*/ , so we can see if an index is used. Also post the table creation scripts (with SHOW CREATE TABLE tableName ). Either this, or you try to create missing indexes yourself.

UPDATE:

Rewritten your query again, might be faster this way:

select
WrappedQuery.*,
regions.name as region_name,
regions.id as region_id,
count(distinct users.id) as nb_users,
count(distinct photos.id) as nb_photos
from (
       select region_id, count(id) VoteCount
          from votes
          where theme_id = '{$currentTheme}'
          group by region_id
          ORDER BY VoteCount DESC
          LIMIT 1
  ) WrappedQuery, regions, users, photos
  WHERE regions.id = WrappedQuery.region_id
  AND users.region_id = WrappedQuery.region_id
  AND photos.region_id = WrappedQuery.region_id
  GROUP BY WrappedQuery.region_id