在这篇文章中，我将重新探究ProxySQL中的Query Rewrite功能，因为query rewriting是创建ProxySQL的最根本初衷。

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为什么我们需要重写查询？

• 你已经确定了一个触发性能瓶颈或导致系统缓慢的查询
• 你无法（快速的）修改应用代码
• 某些特殊的操作需要“重定向查询”

这儿举例你作为DBA发现了一个“坏查询”，你确认是它导致了服务缓慢，并且可能会导致服务不可用。那这个查询必须被优化，你和开发沟通要修正这个SQL，但是开发反馈回来的信息是能改，但是由于技术的非技术的种种原因吧，没有那么快。这时你怎么办，等着？显然不能，你可以在开发完成修正之前通过ProxySQL的Query Rewrite功能重写某些查询来完成优化同时对应用保持透明。

如何重写查询？通过ProxySQL有两种方式来完成（译者注：其实应该理解为两种匹配查询的方式）。

Query rewrite其实就是通过 MySQL_query_rules 表中一个 match_pattern + replace_pattern 的过程，而match_digest （注意区分 match_pattern 和 match_digest ）仅用来匹配一个查询，而非重写它。逻辑上讲，match_digest 和 username，schemaname，proxy_addr 等字段的作用是一样的，仅用来匹配查询。

这两种不同的机制为不同的查询类型（例如DML操作，SELECT等）提供了灵活高效匹配方式。注意如果你希望重写查询，那么规则中的match_pattern必须能匹配到原始的查询。查询规则按照rule_id字段的升序顺序处理，并且只有在active字段为1的前提下才会处理。

下面是我们如何在我们的测试环境演示 match_digest

mysql> SELECT hostgroup hg, sum_time, count_star, digest_text FROM stats_mysql_query_digest ORDER BY sum_time DESC limit 10;
+----+-----------+------------+-----------------------------------+
| hg | sum_time  | count_star | digest_text                       |
+----+-----------+------------+-----------------------------------+
| 0  | 243549572 | 85710      | SELECT c FROM sbtest10 WHERE id=? |
| 0  | 146324255 | 42856      | COMMIT                            |
| 0  | 126643488 | 44310      | SELECT c FROM sbtest7 WHERE id=?  |
| 0  | 126517140 | 42927      | BEGIN                             |
| 0  | 123797307 | 43820      | SELECT c FROM sbtest1 WHERE id=?  |
| 0  | 123345775 | 43460      | SELECT c FROM sbtest6 WHERE id=?  |
| 0  | 122121030 | 43010      | SELECT c FROM sbtest9 WHERE id=?  |
| 0  | 121245265 | 42400      | SELECT c FROM sbtest8 WHERE id=?  |
| 0  | 120554811 | 42520      | SELECT c FROM sbtest3 WHERE id=?  |
| 0  | 119244143 | 42070      | SELECT c FROM sbtest5 WHERE id=?  |
+----+-----------+------------+-----------------------------------+
10 rows in set (0.00 sec)
mysql> INSERT INTO mysql_query_rules (rule_id,active,username,match_digest, match_pattern,replace_pattern,apply) VALUES (10,1,'root','SELECT.*WHERE id=?','sbtest2','sbtest10',1);
Query OK, 1 row affected (0.00 sec)
mysql> LOAD MYSQL QUERY RULES TO RUNTIME;
Query OK, 0 rows affected (0.00 sec)
mysql> SELECT hits, mysql_query_rules.rule_id,digest,active,username, match_digest, match_pattern, replace_pattern, cache_ttl, apply FROM mysql_query_rules NATURAL JOIN stats.stats_mysql_query_rules ORDER BY mysql_query_rules.rule_id;
+------+---------+--------+--------+----------+--------------------+---------------+-----------------+-----------+-------+
| hits | rule_id | digest | active | username | match_digest       | match_pattern | replace_pattern | cache_ttl | apply |
+------+---------+--------+--------+----------+--------------------+---------------+-----------------+-----------+-------+
| 0    | 10      | NULL   | 1      | root     | SELECT.*WHERE id=? | sbtest2       | sbtest10        | NULL      | 1     |
+------+---------+--------+--------+----------+--------------------+---------------+-----------------+-----------+-------+
1 row in set (0.00 sec)
mysql> SELECT hits, mysql_query_rules.rule_id,digest,active,username, match_digest, match_pattern, replace_pattern, cache_ttl, apply FROM mysql_query_rules NATURAL JOIN stats.stats_mysql_query_rules ORDER BY mysql_query_rules.rule_id;
+------+---------+--------+--------+----------+--------------------+---------------+-----------------+-----------+-------+
| hits | rule_id | digest | active | username | match_digest       | match_pattern | replace_pattern | cache_ttl | apply |
+------+---------+--------+--------+----------+--------------------+---------------+-----------------+-----------+-------+
| 593  | 10      | NULL   | 1      | root     | SELECT.*WHERE id=? | sbtest2       | sbtest10        | NULL      | 1     |
+------+---------+--------+--------+----------+--------------------+---------------+-----------------+-----------+-------+
1 row in set (0.00 sec)

如果想清空 query rules 的统计信息，使用下列方法

mysql> SELECT 1 FROM stats_mysql_query_digest_reset LIMIT 1;
+---+
| 1 |
+---+
| 1 |
+---+
1 row in set (0.01 sec)
mysql> LOAD MYSQL QUERY RULES TO RUNTIME;
Query OK, 0 rows affected (0.00 sec)

接下来是 match_pattern 示例：

mysql> SELECT hostgroup hg, sum_time, count_star, digest_text FROM stats_mysql_query_digest ORDER BY sum_time DESC limit 5;
+----+----------+------------+----------------------------------+
| hg | sum_time | count_star | digest_text                      |
+----+----------+------------+----------------------------------+
| 0  | 98753983 | 16292      | BEGIN                            |
| 0  | 84613532 | 16232      | COMMIT                           |
| 1  | 49327292 | 16556      | SELECT c FROM sbtest3 WHERE id=? |
| 1  | 49027118 | 16706      | SELECT c FROM sbtest2 WHERE id=? |
| 1  | 48095847 | 16396      | SELECT c FROM sbtest4 WHERE id=? |
+----+----------+------------+----------------------------------+
5 rows in set (0.01 sec)

mysql> INSERT INTO mysql_query_rules (rule_id,active,username,match_pattern,replace_pattern,apply) VALUES (20,1,'root','DISTINCT(.*)ORDER BY c','DISTINCT1',1);
Query OK, 1 row affected (0.00 sec)

mysql> LOAD MYSQL QUERY RULES TO RUNTIME;
Query OK, 0 rows affected (0.01 sec)

mysql> SELECT hits, mysql_query_rules.rule_id,digest,active,username, match_digest, match_pattern, replace_pattern, cache_ttl, apply FROM mysql_query_rules NATURAL JOIN stats.stats_mysql_query_rules ORDER BY mysql_query_rules.rule_id;
+------+---------+--------+--------+----------+--------------------+------------------------+-----------------+-----------+-------+
| hits | rule_id | digest | active | username | match_digest       | match_pattern          | replace_pattern | cache_ttl | apply |
+------+---------+--------+--------+----------+--------------------+------------------------+-----------------+-----------+-------+
| 0    | 10      | NULL   | 1      | root     | SELECT.*WHERE id=? | sbtest2                | sbtest10        | NULL      | 1     |
| 0    | 20      | NULL   | 1      | root     | NULL               | DISTINCT(.*)ORDER BY c | DISTINCT1      | NULL      | 1     |
+------+---------+--------+--------+----------+--------------------+------------------------+-----------------+-----------+-------+
2 rows in set (0.01 sec)

mysql> SELECT hits, mysql_query_rules.rule_id,digest,active,username, match_digest, match_pattern, replace_pattern, cache_ttl, apply FROM mysql_query_rules NATURAL JOIN stats.stats_mysql_query_rules ORDER BY mysql_query_rules.rule_id;
+------+---------+--------+--------+----------+--------------------+------------------------+-----------------+-----------+-------+
| hits | rule_id | digest | active | username | match_digest       | match_pattern          | replace_pattern | cache_ttl | apply |
+------+---------+--------+--------+----------+--------------------+------------------------+-----------------+-----------+-------+
| 9994 | 10      | NULL   | 1      | root     | SELECT.*WHERE id=? | sbtest2                | sbtest10        | NULL      | 1     |
| 6487 | 20      | NULL   | 1      | root     | NULL               | DISTINCT(.*)ORDER BY c | DISTINCT1      | NULL      | 1     |
+------+---------+--------+--------+----------+--------------------+------------------------+-----------------+-----------+-------+
2 rows in set (0.00 sec)

mysql> SELECT 1 FROM stats_mysql_query_digest_reset LIMIT 1;
+---+
| 1 |
+---+
| 1 |
+---+
1 row in set (0.00 sec)

mysql>  LOAD MYSQL QUERY RULES TO RUNTIME;
Query OK, 0 rows affected (0.00 sec)

路由规则中一个关键点是 mysql_query_rules 的 apply 字段

• apply = 1（默认）表示查询一旦匹配到一条规则就不再匹配剩余的规则
• apply = 0 表示继续尝试匹配后续的规则

（译者注：类似于nginx rewrite 指令中的 break 参数）

如下面测试中所展示的，所有匹配rule_id = 10 或 rule_id = 20 的查询都准确的匹配上了。实际上，现在所有的规则在 runtime_mysql_query_rules 表中都是激活的。如果我们想禁用 mysql_query_rules 表中某条规则，设置 active = 0

mysql> update mysql_query_rules set apply = 1 where rule_id in (10);
Query OK, 1 row affected (0.00 sec)
mysql> update mysql_query_rules set apply = 0 where rule_id in (20);
Query OK, 1 row affected (0.00 sec)
mysql>  LOAD MYSQL QUERY RULES TO RUNTIME;
Query OK, 0 rows affected (0.00 sec)
mysql> SELECT hits, mysql_query_rules.rule_id,digest,active,username, match_digest, match_pattern, replace_pattern, cache_ttl, apply FROM mysql_query_rules NATURAL JOIN stats.stats_mysql_query_rules ORDER BY mysql_query_rules.rule_id;
+------+---------+--------+--------+----------+--------------------+------------------------+-----------------+-----------+-------+
| hits | rule_id | digest | active | username | match_digest       | match_pattern          | replace_pattern | cache_ttl | apply |
+------+---------+--------+--------+----------+--------------------+------------------------+-----------------+-----------+-------+
| 0    | 10      | NULL   | 1      | root     | SELECT.*WHERE id=? | sbtest2                | sbtest10        | NULL      | 1     |
| 0    | 20      | NULL   | 1      | root     | NULL               | DISTINCT(.*)ORDER BY c | DISTINCT1      | NULL      | 0     |
+------+---------+--------+--------+----------+--------------------+------------------------+-----------------+-----------+-------+
2 rows in set (0.00 sec)
mysql> SELECT hits, mysql_query_rules.rule_id,digest,active,username, match_digest, match_pattern, replace_pattern, flagIN, apply FROM mysql_query_rules NATURAL JOIN stats.stats_mysql_query_rules ORDER BY mysql_query_rules.rule_id;
+-------+---------+--------+--------+----------+--------------------+------------------------+-----------------+--------+-------+
| hits  | rule_id | digest | active | username | match_digest       | match_pattern          | replace_pattern | flagIN | apply |
+-------+---------+--------+--------+----------+--------------------+------------------------+-----------------+--------+-------+
| 10195 | 10      | NULL   | 1      | root     | SELECT.*WHERE id=? | sbtest2                | sbtest10        | 0      | 1     |
| 6599  | 20      | NULL   | 1      | root     | NULL               | DISTINCT(.*)ORDER BY c | DISTINCT1      | 0      | 0     |
+-------+---------+--------+--------+----------+--------------------+------------------------+-----------------+--------+-------+
2 rows in set (0.00 sec)
mysql> SELECT hits, mysql_query_rules.rule_id,digest,active,username, match_digest, match_pattern, replace_pattern, flagIN, apply FROM mysql_query_rules NATURAL JOIN stats.stats_mysql_query_rules ORDER BY mysql_query_rules.rule_id;
+-------+---------+--------+--------+----------+--------------------+------------------------+-----------------+--------+-------+
| hits  | rule_id | digest | active | username | match_digest       | match_pattern          | replace_pattern | flagIN | apply |
+-------+---------+--------+--------+----------+--------------------+------------------------+-----------------+--------+-------+
| 20217 | 5       | NULL   | 1      | root     | NULL               | DISTINCT(.*)ORDER BY c | DISTINCT1      | 0      | 1     |
| 27020 | 10      | NULL   | 1      | root     | SELECT.*WHERE id=? | sbtest2                | sbtest10        | 0      | 0     |
+-------+---------+--------+--------+----------+--------------------+------------------------+-----------------+--------+-------+
2 rows in set (0.00 sec)
mysql> update mysql_query_rules set active = 0 where rule_id = 5;
Query OK, 1 row affected (0.00 sec)
mysql>  LOAD MYSQL QUERY RULES TO RUNTIME;
Query OK, 0 rows affected (0.02 sec)
mysql> SELECT hits, mysql_query_rules.rule_id,digest,active,username, match_digest, match_pattern, replace_pattern, cache_ttl, apply FROM mysql_query_rules NATURAL JOIN stats.stats_mysql_query_rules ORDER BY mysql_query_rules.rule_id;
+------+---------+--------+--------+----------+--------------------+---------------+-----------------+-----------+-------+
| hits | rule_id | digest | active | username | match_digest       | match_pattern | replace_pattern | cache_ttl | apply |
+------+---------+--------+--------+----------+--------------------+---------------+-----------------+-----------+-------+
| 0    | 10      | NULL   | 1      | root     | SELECT.*WHERE id=? | sbtest2       | sbtest10        | NULL      | 0     |
+------+---------+--------+--------+----------+--------------------+---------------+-----------------+-----------+-------+
1 row in set (0.00 sec)
mysql> SELECT hits, mysql_query_rules.rule_id,digest,active,username, match_digest, match_pattern, replace_pattern, cache_ttl, apply FROM mysql_query_rules NATURAL JOIN stats.stats_mysql_query_rules ORDER BY mysql_query_rules.rule_id;
+------+---------+--------+--------+----------+--------------------+---------------+-----------------+-----------+-------+
| hits | rule_id | digest | active | username | match_digest       | match_pattern | replace_pattern | cache_ttl | apply |
+------+---------+--------+--------+----------+--------------------+---------------+-----------------+-----------+-------+
| 4224 | 10      | NULL   | 1      | root     | SELECT.*WHERE id=? | sbtest2       | sbtest10        | NULL      | 0     |
+------+---------+--------+--------+----------+--------------------+---------------+-----------------+-----------+-------+
1 row in set (0.01 sec)

另外，ProxySQL还能帮忙识别出“低效的查询”，登录管理界面按如下操作
找出总耗时最多的查询

mysql> SELECT SUM(sum_time), SUM(count_star), digest_text FROM stats_mysql_query_digest GROUP BY digest ORDER BY SUM(sum_time) DESC LIMIT 3G
*************************** 1. row ***************************
  SUM(sum_time): 95053795
SUM(count_star): 13164
    digest_text: BEGIN
*************************** 2. row ***************************
  SUM(sum_time): 85094367
SUM(count_star): 13130
    digest_text: COMMIT
*************************** 3. row ***************************
  SUM(sum_time): 52110099
SUM(count_star): 13806
    digest_text: SELECT c FROM sbtest3 WHERE id=?
3 rows in set (0.00 sec)

找出平均耗时最高的查询

mysql> SELECT SUM(sum_time), SUM(count_star), SUM(sum_time)/SUM(count_star) avg,  digest_text FROM stats_mysql_query_digest GROUP BY digest ORDER BY SUM(sum_time)/SUM(count_star) DESC limit 1;
+---------------+-----------------+--------+--------------------------------+
| SUM(sum_time) | SUM(count_star) | avg    | digest_text                    |
+---------------+-----------------+--------+--------------------------------+
| 972162        | 1               | 972162 | CREATE INDEX k_5 ON sbtest5(k) |
+---------------+-----------------+--------+--------------------------------+
1 row in set (0.00 sec)

我发现关于ProxySQL query rewrite 的“最好”的文档在IBM，这里介绍了查询重写的原理和示例，值得一读。

还有一些别的场景你可能需要重写查询，试想有一张表的自增ID列已经达到了int类型的最大值，你可以将新插入的数据重定向到另一张表同时你通过alter命令来修正原表的问题，在这期间所有的查询还将访问原表，等alter原表完成后，将新表的数据导入的原表，即可达到不停机修DDL的效果。

从MySQL 5.7.6 起，MySQL以插件形式提供了 query rewrite 功能，你可以在这里找到相关文档。MySQL内建的查询重写功能的一个最大的劣势在于重写规则仅作用于当前MySQL实例，这也是相比之下ProxySQL 的优势所在：它处在应用和数据库之间，所以它的重写规则是全局的。

原文链接

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