为什么此查询不对ORDER BY使用索引？

Question

SELECT `f`.*
FROM `files_table` `f`
WHERE f.`application_id` IN(6)
AND `f`.`project_id` IN(130418)
AND `f`.`is_last_version` = 1
AND `f`.`temporary` = 0
AND f.deleted_by is null
ORDER BY `f`.`date` DESC
LIMIT 5

When I remove the ORDER BY, query executes in 0.1 seconds. With the ORDER BY it takes 3 seconds.

There is an index on every WHERE column and there is also an index on ORDER BY field (date).

What can I do to make this query faster? Why is ORDER BY slowing it down so much? Table has 3M rows.

Answer 1

instead of an index on each column in where be sure you have a composite index that cover all the columns in where

eg

create index  idx1 on files_table (application_id, project_id,is_last_version,temporary,deleted_by)

avoid IN clause for single value use = for these

  SELECT `f`.*
  FROM `files_table` `f`
  WHERE f.`application_id`  = 6 
  AND `f`.`project_id` = 130418
  AND `f`.`is_last_version` = 1
  AND `f`.`temporary` = 0
  AND f.deleted_by is null
  ORDER BY `f`.`date` DESC
  LIMIT 5

the date or others column in select could be useful retrive all info using the index and avoiding the access to the table data .. but for select all (select *) you probably need severl columns an then the access to the table data is done however .. but you can try an eval the performance ..

be careful to place the data non involved in where at the right of all the column involved in where

create index  idx1 on files_table (application_id, project_id,is_last_version,temporary,deleted_by, date)