mysql - 按日期时间排序时，大量连接的 MySQL 查询会变慢，而按 ID 排序则不会变慢

Question

我有一个 MySQL(实际上是 MariaDB)查询，当我尝试按 DATETIME 字段(sighted)而不是主键排序时，该查询运行速度要慢得多，并且会回退到文件排序。我在 sighted 上有一个索引，但由于某种原因它似乎没有使用它。如何为该表建立索引，以便可以按 DATETIME 字段有效地对其进行排序？

select 
    user_id , t.name, f.value, y.name 
    from posts p 
        left join taxon_sightings t on (t.post_id = p.id) 
        left join fields f on (f.foreign_id = t.id) 
        left join fieldtypes y on (y.id = f.fieldtype_id) 
    group by p.id 
    order by p.sighted desc 
    limit 10;

当我按sighted DATETIME 字段排序时:

MariaDB [dbase]> explain [yadda yadda, as above]
+----+-------------+-------+--------+---------------+------------+---------+--------------------------------+------+---------------------------------+
| id | select_type | table | type   | possible_keys | key        | key_len | ref                            | rows | Extra                           |
+----+-------------+-------+--------+---------------+------------+---------+--------------------------------+------+---------------------------------+
|  1 | SIMPLE      | p     | ALL    | NULL          | NULL       | NULL    | NULL                           | 1759 | Using temporary; Using filesort |
|  1 | SIMPLE      | t     | ref    | post_id       | post_id    | 4       | dataportal_test.p.id           |   12 |                                 |
|  1 | SIMPLE      | f     | ref    | foreign_id    | foreign_id | 4       | dataportal_test.t.id           |    6 | Using where                     |
|  1 | SIMPLE      | y     | eq_ref | PRIMARY       | PRIMARY    | 4       | dataportal_test.f.fieldtype_id |    1 | Using where                     |
+----+-------------+-------+--------+---------------+------------+---------+--------------------------------+------+---------------------------------+

与按 ID 排序时相比:

+----+-------------+-------+--------+---------------+------------+---------+--------------------------------+------+-------------+
| id | select_type | table | type   | possible_keys | key        | key_len | ref                            | rows | Extra       |
+----+-------------+-------+--------+---------------+------------+---------+--------------------------------+------+-------------+
|  1 | SIMPLE      | p     | index  | NULL          | PRIMARY    | 4       | NULL                           |    1 |             |
|  1 | SIMPLE      | t     | ref    | post_id       | post_id    | 4       | dataportal_test.p.id           |   12 |             |
|  1 | SIMPLE      | f     | ref    | foreign_id    | foreign_id | 4       | dataportal_test.t.id           |    6 | Using where |
|  1 | SIMPLE      | y     | eq_ref | PRIMARY       | PRIMARY    | 4       | dataportal_test.f.fieldtype_id |    1 | Using where |
+----+-------------+-------+--------+---------------+------------+---------+--------------------------------+------+-------------+

Answer 1

我认为你做不到。我能想到的唯一技巧就是尝试这个:

select user_id, t.name, f.value, y.name 
from posts p left join
     taxon_sightings t
     on (t.post_id = p.id) left join
     fields f
     on (f.foreign_id = t.id) left join
     fieldtypes y
     on (y.id = f.fieldtype_id) 
group by p.sighted, p.id 
---------^ very important
order by p.sighted desc 
limit 10;

然后在 posts(sighted, id) 上添加索引。 group by 和 order by 可能会利用索引。如果你幸运的话。 。 .