Question

简而言之，有两个表，activities_log有330万行，posts有20K行。当一些查询加入他们的过程超过15秒！（╥﹏╥）

我做错了什么？我可以做些什么来优化？

它正在此服务器上运行：

适用于Linux（x86_64）的MySQL版本14.14 Distrib 5.7.26，使用EditLine包装器
数字海洋小滴/ 4 GB内存（4 GB交换）/ 60 GB磁盘
Ubuntu 18.04.1 LTS（GNU / Linux 4.15.0-43-通用x86_64）

# QUERY:
select `posts`.`page_id` from `activities_log` left join `posts` on `posts`.`id` = `activities_log`.`post_id`;
3345753 rows in set (17.40 sec)

# EXPLAIN:
+----+-------------+----------------+------------+--------+---------------+------------------------------+---------+------------------------------------+---------+----------+-------------+
| id | select_type | table          | partitions | type   | possible_keys | key                          | key_len | ref                                | rows    | filtered | Extra       |
+----+-------------+----------------+------------+--------+---------------+------------------------------+---------+------------------------------------+---------+----------+-------------+
|  1 | SIMPLE      | activities_log | NULL       | index  | NULL          | activities_log_post_id_index | 145     | NULL                               | 3203032 |   100.00 | Using index |
|  1 | SIMPLE      | posts          | NULL       | eq_ref | PRIMARY       | PRIMARY                      | 144     | prod_api_v1.activities_log.post_id |       1 |   100.00 | NULL        |
+----+-------------+----------------+------------+--------+---------------+------------------------------+---------+------------------------------------+---------+----------+-------------+
2 rows in set, 1 warning (0.01 sec)

select count(*) from `activities_log`;
+----------+
| count(*) |
+----------+
|  3345770 |
+----------+
1 row in set (1.04 sec)

show index from activities_log;
+----------------+------------+------------------------------------+--------------+---------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table          | Non_unique | Key_name                           | Seq_in_index | Column_name   | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+----------------+------------+------------------------------------+--------------+---------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| activities_log |          0 | PRIMARY                            |            1 | id            | A         |     2984883 |     NULL | NULL   |      | BTREE      |         |               |
| activities_log |          1 | activities_log_page_id_index       |            1 | page_id       | A         |         343 |     NULL | NULL   | YES  | BTREE      |         |               |
| activities_log |          1 | activities_log_activity_id_index   |            1 | activity_id   | A         |          15 |     NULL | NULL   |      | BTREE      |         |               |
| activities_log |          1 | activities_log_post_id_index       |            1 | post_id       | A         |       43894 |     NULL | NULL   | YES  | BTREE      |         |               |
| activities_log |          1 | activities_log_session_token_index |            1 | session_token | A         |        4431 |     NULL | NULL   | YES  | BTREE      |         |               |
| activities_log |          1 | activities_log_user_id_index       |            1 | user_id       | A         |       17908 |     NULL | NULL   | YES  | BTREE      |         |               |
+----------------+------------+------------------------------------+--------------+---------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
6 rows in set (0.00 sec)

select count(*) from `posts`;
+----------+
| count(*) |
+----------+
|    19999 |
+----------+
1 row in set (0.00 sec)

show index from posts;
+-------+------------+--------------------------+--------------+--------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name                 | Seq_in_index | Column_name  | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------+------------+--------------------------+--------------+--------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| posts |          0 | PRIMARY                  |            1 | id           | A         |       16647 |     NULL | NULL   |      | BTREE      |         |               |
| posts |          1 | posts_page_id_index      |            1 | page_id      | A         |         324 |     NULL | NULL   |      | BTREE      |         |               |
| posts |          1 | posts_kind_post_id_index |            1 | kind_post_id | A         |           8 |     NULL | NULL   |      | BTREE      |         |               |
| posts |          1 | posts_posted_by_index    |            1 | posted_by    | A         |          31 |     NULL | NULL   | YES  | BTREE      |         |               |
+-------+------------+--------------------------+--------------+--------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
4 rows in set (0.00 sec)

编辑：

此查询仅是示例。我真的不想要页面ID。一个真实的例子可能是：select * from activities_log left join posts on posts.id = activities_log.post_id where activities_log.page_id = X or posts.page_id = X

Answer 1

应该为每个列使用复合冗余索引，而不是为每个列使用单独的索引

表

  activities_log    column  ( post_id, page_id)

在post_id左位置（join子句中涉及的列），在page_id后面。。此列对于避免访问表数据并从索引表获取所有数据很有用

您应该记住，查询只能为每个涉及的表使用单个indexc

Answer 2

我不知道您要完成什么。您有一个left join，并且正在从 second 表返回行，因此这些行通常是NULL。

尽管有您的疑问，我的最佳猜测是您希望所有具有活动的帖子的页面。在这种情况下，您可以将查询的短语设置为：

select p.page_id
from posts p
where exists (select 1
              from activities_log al
              where p.id = al.post_id
             );

至少，这将返回更小的结果集。

MySQL JOIN需要10秒钟以上的时间

2 个答案: