Question

我在MySQL数据库中有表格，其中> 20 000 000行，下面的查询在少量行上执行很好，但如果有更多行则需要2-3秒。如何优化它以使其运行＆lt;至少1？注意 - 问题出在子查询SELECT read_state FROM messages ... 查询：

SELECT sql_no_cache users.id AS uid,
  name,
  avatar,
  avatar_date,
  driver,
  msg,
  DATE,
  messages.removed,
  from_id = 528798 AS outbox ,
  !(0    IN
  (SELECT read_state
  FROM messages AS msgs FORCE KEY(user_id_2)
  WHERE (msgs.from_id = messages.from_id
  OR msgs.from_id = messages.user_id)
  AND msgs.user_id = 528798
  AND removed = 0
  )) AS read_state
FROM dialog,
  messages,
  users
WHERE messages.id = mid
AND ((uid1 = 528798
AND users.id = uid2)
OR (uid2 = 528798
AND users.id = uid1))
ORDER BY DATE DESC;

显示消息中的索引;

+----------+------------+-------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table    | Non_unique | Key_name    | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+----------+------------+-------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| messages |          0 | PRIMARY     |            1 | id          | A         |    27531939 |     NULL | NULL   |      | BTREE      |         |               |
| messages |          1 | to_number   |            1 | to_number   | A         |          22 |     NULL | NULL   |      | BTREE      |         |               |
| messages |          1 | from_id     |            1 | from_id     | A         |      529460 |     NULL | NULL   |      | BTREE      |         |               |
| messages |          1 | from_id     |            2 | to_number   | A         |      529460 |     NULL | NULL   |      | BTREE      |         |               |
| messages |          1 | user_id_2   |            1 | user_id     | A         |      655522 |     NULL | NULL   |      | BTREE      |         |               |
| messages |          1 | user_id_2   |            2 | read_state  | A         |      917731 |     NULL | NULL   |      | BTREE      |         |               |
| messages |          1 | user_id_2   |            3 | removed     | A         |      949377 |     NULL | NULL   |      | BTREE      |         |               |
| messages |          1 | idx_user_id |            1 | user_id     | A         |      809762 |     NULL | NULL   |      | BTREE      |         |               |
| messages |          1 | idx_from_id |            1 | from_id     | A         |      302548 |     NULL | NULL   |      | BTREE      |         |               |
+----------+------------+-------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+

desc消息;

+------------+-------------+------+-----+---------+----------------+
| Field      | Type        | Null | Key | Default | Extra          |
+------------+-------------+------+-----+---------+----------------+
| id         | int(11)     | NO   | PRI | NULL    | auto_increment |
| from_id    | int(11)     | NO   | MUL | NULL    |                |
| user_id    | int(11)     | NO   | MUL | NULL    |                |
| group_id   | int(11)     | NO   |     | NULL    |                |
| to_number  | varchar(30) | NO   | MUL | NULL    |                |
| msg        | text        | NO   |     | NULL    |                |
| image      | varchar(20) | NO   |     | NULL    |                |
| date       | bigint(20)  | NO   |     | NULL    |                |
| read_state | tinyint(1)  | NO   |     | 0       |                |
| removed    | tinyint(1)  | NO   |     | NULL    |                |
+------------+-------------+------+-----+---------+----------------+

EXPLAIN EXTENDED：

+----+--------------------+----------+-------------+---------------+-----------+---------+--------------------+--------+----------+---------------------------------------------------------------------------+
| id | select_type        | table    | type        | possible_keys | key       | key_len | ref                | rows   | filtered | Extra                                                                     |
+----+--------------------+----------+-------------+---------------+-----------+---------+--------------------+--------+----------+---------------------------------------------------------------------------+
|  1 | PRIMARY            | dialog   | index_merge | uid1,uid2     | uid1,uid2 | 4,4     | NULL               |   1707 |   100.00 | Using sort_union(uid1,uid2); Using where; Using temporary; Using filesort |
|  1 | PRIMARY            | users    | ALL         | PRIMARY       | NULL      | NULL    | NULL               | 608993 |   100.00 | Range checked for each record (index map: 0x1)                            |
|  1 | PRIMARY            | messages | eq_ref      | PRIMARY       | PRIMARY   | 4       | numbers.dialog.mid |      1 |   100.00 |                                                                           |
|  2 | DEPENDENT SUBQUERY | msgs     | ref         | user_id_2     | user_id_2 | 6       | const,const,const  |   2607 |   100.00 | Using where                                                               |
+----+--------------------+----------+-------------+---------------+-----------+---------+--------------------+--------+----------+---------------------------------------------------------------------------+

Answer 1

做出一些猜测，这样的事情可能更有效： -

SELECT DISTINCT users.id AS uid,
  name,
  avatar,
  avatar_date,
  driver,
  msg,
  `DATE`,
  messages.removed,
  from_id = 528798 AS outbox ,
  CASE WHEN msgs.read_state IS NULL THEN 1 ELSE 0 END AS read_state
FROM messages
INNER JOIN dialog ON messages.id = dialog.mid
INNER JOIN users ON (dialog.uid1 = 528798 AND users.id = dialog.uid2) OR (dialog.uid2 = 528798 AND users.id = dialog.uid1)
LEFT OUTER JOIN messages msgs ON msgs.read_state = 0 AND msgs.user_id = 528798 AND removed = 0 AND (msgs.from_id = messages.from_id OR msgs.from_id = messages.user_id)
ORDER BY `DATE` DESC;

这是再次对消息进行LEFT JOIN的额外连接，然后使用case将结果转换为0或1。

当LEFT JOIN可以带回多个匹配的行时，DISTINCT应该应对（如果不可能，那么你可以消除DISTINCT）

怀疑加入用户的OR条款效率不高。用2 LEFT OUTER JOIN替换INNER JOIN可能更好。这样的事情： -

SELECT DISTINCT COALESCE(users1.id, users2.id) AS uid,
  COALESCE(users1.name, users2.name),
  COALESCE(users1.avatar, users2.avatar),
  COALESCE(users1.avatar_date, users2.avatar_date),
  COALESCE(users1.driver, users2.driver),
  msg,
  `DATE`,
  messages.removed,
  from_id = 528798 AS outbox ,
  CASE WHEN msgs.read_state IS NULL THEN 1 ELSE 0 END AS read_state
FROM messages
INNER JOIN dialog ON messages.id = dialog.mid
LEFT OUTER JOIN users users1 ON (dialog.uid1 = 528798 AND users1.id = dialog.uid2)
LEFT OUTER JOIN users users2 ON (dialog.uid2 = 528798 AND users2.id = dialog.uid1)
LEFT OUTER JOIN messages msgs ON msgs.read_state = 0 AND msgs.user_id = 528798 AND removed = 0 AND (msgs.from_id = messages.from_id OR msgs.from_id = messages.user_id)
WHERE users1.id IS NOT NULL
OR users2.id IS NOT NULL
ORDER BY `DATE` DESC;

不在子查询MYSQL中进行优化

1 个答案: