使用
FreeBSD freebsd 13.2-RELEASE-p2 FreeBSD 13.2-RELEASE-p2 GENERIC amd64mariadb104-server-10.4.28mysql Ver 15.1 Distrib 10.5.20-MariaDB, for FreeBSD13.2 (amd64) using EditLine wrapper试图找出为什么查询：
select v.id, b.book_number, b.title, v.question_ocr
from version v
join solution s on v.solution_id = s.id
join assignment a on s.assignment_id = a.id
join book b on a.book_id = b.id
order by v.created_at desc limit 10;
执行时间超过 3 秒。
解释显示：
+------+-------------+-------+--------+------------------------------+----------------------+---------+-----------------------+------+-----------------------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+------+-------------+-------+--------+------------------------------+----------------------+---------+-----------------------+------+-----------------------------------------------------------+
| 1 | SIMPLE | a | index | PRIMARY,IDX_30C544BA16A2B381 | IDX_30C544BA16A2B381 | 145 | NULL | 1344 | Using where; Using index; Using temporary; Using filesort |
| 1 | SIMPLE | b | eq_ref | PRIMARY | PRIMARY | 144 | db_name____.a.book_id | 1 | |
| 1 | SIMPLE | s | ref | PRIMARY,IDX_9F3329DBD19302F8 | IDX_9F3329DBD19302F8 | 145 | db_name____.a.id | 35 | Using index |
| 1 | SIMPLE | v | ref | IDX_BF1CD3C31C0BE183 | IDX_BF1CD3C31C0BE183 | 145 | db_name____.s.id | 1 | |
+------+-------------+-------+--------+------------------------------+----------------------+---------+-----------------------+------+-----------------------------------------------------------+
有趣的是，当我删除
order by我尝试了 vmstat、iostat、top、netstat 等分析工具。
有没有人可以解决查询速度慢的问题？
created_at描述
version+---------------------+--------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+---------------------+--------------+------+-----+---------+-------+
| id | char(36) | NO | PRI | NULL | |
| solution_id | char(36) | YES | MUL | NULL | |
| status | varchar(255) | NO | | NULL | |
| created_at | datetime | NO | MUL | NULL | |
+---------------------+--------------+------+-----+---------+-------+
Solution+------------------+--------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+------------------+--------------+------+-----+---------+-------+
| id | char(36) | NO | PRI | NULL | |
| book_id | char(36) | YES | MUL | NULL | |
| exercise_number | varchar(255) | NO | | NULL | |
| created_at | datetime | NO | | NULL | |
| assignment_id | char(36) | YES | MUL | NULL | |
+------------------+--------------+------+-----+---------+-------+
Assignment+--------------------------------+--------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+--------------------------------+--------------+------+-----+---------+-------+
| id | char(36) | NO | PRI | NULL | |
| book_id | char(36) | YES | MUL | NULL | |
| price | double | YES | | NULL | |
| created_at | datetime | YES | | NULL | |
+--------------------------------+--------------+------+-----+---------+-------+
Book+--------------------------+--------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+--------------------------+--------------+------+-----+---------+-------+
| id | char(36) | NO | PRI | NULL | |
| title | varchar(255) | NO | | NULL | |
| book_number | varchar(255) | NO | UNI | NULL | |
+--------------------------+--------------+------+-----+---------+-------+
version+---------+------------+----------------------------+--------------+---------------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+---------+------------+----------------------------+--------------+---------------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| version | 0 | PRIMARY | 1 | id | A | 223710 | NULL | NULL | | BTREE | | |
| version | 1 | IDX_BF1CD3C31C0BE183 | 1 | solution_id | A | 223710 | NULL | NULL | YES | BTREE | | |
| version | 1 | idx_solution_id_created_at | 1 | solution_id | A | 223710 | NULL | NULL | YES | BTREE | | |
| version | 1 | idx_solution_id_created_at | 2 | created_at | A | 223710 | NULL | NULL | | BTREE | | |
+---------+------------+----------------------------+--------------+---------------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
Solution+----------+------------+----------------------+--------------+------------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+----------+------------+----------------------+--------------+------------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| solution | 0 | PRIMARY | 1 | id | A | 91914 | NULL | NULL | | BTREE | | |
| solution | 1 | IDX_9F3329DB16A2B381 | 1 | book_id | A | 666 | NULL | NULL | YES | BTREE | | |
| solution | 1 | IDX_9F3329DBD19302F8 | 1 | assignment_id | A | 2872 | NULL | NULL | YES | BTREE | | |
| solution | 1 | IDX_9F3329DB4B09E92C | 1 | administrator_id | A | 154 | NULL | NULL | YES | BTREE | | |
+----------+------------+----------------------+--------------+------------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
Assignment+------------+------------+-----------------------+--------------+----------------------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+------------+------------+-----------------------+--------------+----------------------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| assignment | 0 | PRIMARY | 1 | id | A | 1431 | NULL | NULL | | BTREE | | |
| assignment | 1 | IDX_30C544BA16A2B381 | 1 | book_id | A | 715 | NULL | NULL | YES | BTREE | | |
| assignment | 1 | IDX_30C544BA4B09E92C | 1 | administrator_id | A | 130 | NULL | NULL | YES | BTREE | | |
+------------+------------+-----------------------+--------------+----------------------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
Book+-------+------------+-----------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------+------------+-----------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| book | 0 | PRIMARY | 1 | id | A | 2698 | NULL | NULL | | BTREE | | |
| book | 1 | IDX_CBE5A33123EDC87 | 1 | subject_id | A | 42 | NULL | NULL | YES | BTREE | | |
+-------+------------+-----------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
my.cnfsql_mode = STRICT_TRANS_TABLES,NO_ZERO_DATE,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION,NO_ZERO_IN_DATE,ERROR_FOR_DIVISION_BY_ZERO
bind-address = 127.0.0.1
socket = /var/run/mysql/mysql.sock
innodb_print_all_deadlocks = ON
collation_server = utf8_unicode_ci
character_set_server = utf8
innodb_buffer_pool_size = 8G
performance_schema = ON
innodb_thread_concurrency = 10
max_connections = 150
innodb_flush_log_at_trx_commit = 2
join_buffer_size = 512K
key_buffer_size = 128M
sort_buffer_size = 1M
您是否尝试将created_at中的索引更改为按DESC而不是ASC排序？ 您可以在这里阅读更多相关内容： https://dev.mysql.com/doc/refman/8.0/en/descending-indexes.html