Question

Here is my query

SELECT file_id, file_name, file_date, file_email
FROM (SELECT *
      FROM `file`
      ORDER BY file_date DESC
     ) AS t
WHERE file_domains = ''
GROUP BY file_name
ORDER BY file_date DESC
LIMIT 0 , 100

primary key is file_id and index is file_name. Records about 900k
It took about 2 seconds in my local computer.
Is there any optimize for this query?
thanks in advance.

Answer 1

Your query uses a non-standard “feature” (mistake: one non-standard and one semi-standard feature) of MySQL and there is no guarantee that it will not break in future versions of MySQL, when the optimizer will be clever enough to understand that the subquery is redundant.

Add an index on (file_domains, file_name, file_date) and try this version:

SELECT f.file_id, f.file_name, f.file_date, f.file_email
FROM 
   `file` AS f
  JOIN
    ( SELECT file_name
          ,  MAX(file_date) AS max_file_date
      FROM `file`
      WHERE file_domains = ''
      GROUP BY file_name
      ORDER BY max_file_date DESC
      LIMIT 0 , 100
     ) AS fm
  ON  fm.file_name = f.file_name
  AND fm.max_file_date = f.file_date
ORDER BY f.file_date DESC ;