生产环境中，SQL语句的执行计划经常会发生改变，导致数据库不稳定。阿里云利用Optimizer Hint和Index Hint让MySQL稳定执行计划，该方法称为Statement Outline，并提供了工具包（DBMS_OUTLN）便于您快捷使用。

前提条件

RDS实例版本如下：

  • MySQL 8.0
  • MySQL 5.7

功能设计

Statement Outline支持官方MySQL 8.0、MySQL 5.7的所有hint类型，分为如下两类：

  • Optimizer Hint

    根据作用域和hint对象，分为Global level hint、Table/Index level hint、Join order hint等。详情请参见MySQL官网。

  • Index Hint

    根据Index Hint的类型和范围进行分类。详情请参见MySQL官网

Statement Outline表介绍

AliSQL内置了一个系统表（outline）保存hint，系统启动时会自动创建该表，无需您手动创建。这里提供表的创建语句供您参考： 
​CREATE TABLE `mysql`.`outline` (
  `Id` bigint(20) NOT NULL AUTO_INCREMENT,
  `Schema_name` varchar(64) COLLATE utf8_bin DEFAULT NULL,
  `Digest` varchar(64) COLLATE utf8_bin NOT NULL,
  `Digest_text` longtext COLLATE utf8_bin,
  `Type` enum('IGNORE INDEX','USE INDEX','FORCE INDEX','OPTIMIZER') CHARACTER SET utf8 COLLATE utf8_general_ci NOT NULL,
  `Scope` enum('','FOR JOIN','FOR ORDER BY','FOR GROUP BY') CHARACTER SET utf8 COLLATE utf8_general_ci DEFAULT '',
  `State` enum('N','Y') CHARACTER SET utf8 COLLATE utf8_general_ci NOT NULL DEFAULT 'Y',
  `Position` bigint(20) NOT NULL,
  `Hint` text COLLATE utf8_bin NOT NULL,
  PRIMARY KEY (`Id`)
) /*!50100 TABLESPACE `mysql` */ ENGINE=InnoDB
DEFAULT CHARSET=utf8 COLLATE=utf8_bin STATS_PERSISTENT=0 COMMENT='Statement outline'​

参数说明如下。

参数 说明
Id Outline ID。
Schema_name 数据库名。
Digest Digest_text进行hash计算得到的64字节的hash字符串。
Digest_text SQL语句的特征。
Type
  • Optimizer Hint中，hint类型的取值为OPTIMIZER。
  • Index Hint中，hint类型的取值为USE INDEX、FORCE INDEX或IGNORE INDEX。
Scope 仅Index Hint需要提供，分为如下三类：
  • FOR GROUP BY
  • FOR ORDER BY
  • FOR JOIN

空串表示所有类型的Index Hint。
State 本规则是否启用。
Position
  • Optimizer Hint中，Position表示Query Block， 因为所有的Optimizer Hint必须作用到 Query Block上，所以，Position从1开始，hint作用在语句的第几个关键字上，Position就是几。
  • Index Hint中，Position表示表的位置， 也是从1开始，hint作用在第几个表上，Position就是几。
Hint
  • Optimizer Hint中，Hint表示完整的hint字符串，例如/*+ MAX_EXECUTION_TIME(1000) */。
  • Index Hint中，Hint表示索引名字的列表， 例如ind_1,ind_2。

管理Statement Outline

为了便捷地管理Statement Outline，AliSQL在DBMS_OUTLN中定义了六个本地存储规则。详细说明如下：
  • add_optimizer_outline
    增加Optimizer Hint。命令如下： 
    dbms_outln.add_optimizer_outline('<Schema_name>','<Digest>','<query_block>','<hint>','<query>');
    说明 Digest和Query（原始SQL语句）可以任选其一。如果填写Query，DBMS_OUTLN会计算Digest和Digest_text。

    示例：

    mysql> call dbms_outln.add_optimizer_outline("outline_db", '', 1, '/*+ MAX_EXECUTION_TIME(1000) */',
                                      "select * from t1 where id = 1");
  • add_index_outline
    增加Index Hint。命令如下： 
    dbms_outln.add_index_outline('<Schema_name>','<Digest>',<Position>,'<Type>','<Hint>','<Scope>','<Query>');
    说明 Digest和Query（原始SQL语句）可以任选其一。如果填写Query，DBMS_OUTLN会计算Digest和Digest_text。

    示例：

    mysql> call dbms_outln.add_index_outline('outline_db', '', 1, 'USE INDEX', 'ind_1', '',
                                "select * from t1 where t1.col1 =1 and t1.col2 ='xpchild'");
  • preview_outline
    查看匹配Statement Outline的情况，可用于手动验证。命令如下： 
    dbms_outln.preview_outline('<Schema_name>','<Query>');

    示例：

    ​mysql> call dbms_outln.preview_outline('outline_db', "select * from t1 where t1.col1 =1 and t1.col2 ='xpchild'");
+------------+------------------------------------------------------------------+------------+------------+-------+---------------------+
| SCHEMA     | DIGEST                                                           | BLOCK_TYPE | BLOCK_NAME | BLOCK | HINT                |
+------------+------------------------------------------------------------------+------------+------------+-------+---------------------+
| outline_db | b4369611be7ab2d27c85897632576a04bc08f50b928a1d735b62d0a140628c4c | TABLE      | t1         |     1 | USE INDEX (`ind_1`) |
+------------+------------------------------------------------------------------+------------+------------+-------+---------------------+
1 row in set (0.00 sec)​
  • show_outline
    展示Statement Outline在内存中命中的情况。命令如下： 
    dbms_outln.show_outline();

    示例：

    ​mysql> call dbms_outln.show_outline();
+------+------------+------------------------------------------------------------------+-----------+-------+------+-------------------------------------------------------+------+----------+-------------------------------------------------------------------------------------+
| ID   | SCHEMA     | DIGEST                                                           | TYPE      | SCOPE | POS  | HINT                                                  | HIT  | OVERFLOW | DIGEST_TEXT                                                                         |
+------+------------+------------------------------------------------------------------+-----------+-------+------+-------------------------------------------------------+------+----------+-------------------------------------------------------------------------------------+
|   33 | outline_db | 36bebc61fce7e32b93926aec3fdd790dad5d895107e2d8d3848d1c60b74bcde6 | OPTIMIZER |       |    1 | /*+ SET_VAR(foreign_key_checks=OFF) */                |    1 |        0 | SELECT * FROM `t1` WHERE `id` = ?                                                   |
|   32 | outline_db | 36bebc61fce7e32b93926aec3fdd790dad5d895107e2d8d3848d1c60b74bcde6 | OPTIMIZER |       |    1 | /*+ MAX_EXECUTION_TIME(1000) */                       |    2 |        0 | SELECT * FROM `t1` WHERE `id` = ?                                                   |
|   34 | outline_db | d4dcef634a4a664518e5fb8a21c6ce9b79fccb44b773e86431eb67840975b649 | OPTIMIZER |       |    1 | /*+ BNL(t1,t2) */                                     |    1 |        0 | SELECT `t1` . `id` , `t2` . `id` FROM `t1` , `t2`                                   |
|   35 | outline_db | 5a726a609b6fbfb76bb8f9d2a24af913a2b9d07f015f2ee1f6f2d12dfad72e6f | OPTIMIZER |       |    2 |  /*+ QB_NAME(subq1) */                                |    2 |        0 | SELECT * FROM `t1` WHERE `t1` . `col1` IN ( SELECT `col1` FROM `t2` )               |
|   36 | outline_db | 5a726a609b6fbfb76bb8f9d2a24af913a2b9d07f015f2ee1f6f2d12dfad72e6f | OPTIMIZER |       |    1 | /*+ SEMIJOIN(@subq1 MATERIALIZATION, DUPSWEEDOUT) */  |    2 |        0 | SELECT * FROM `t1` WHERE `t1` . `col1` IN ( SELECT `col1` FROM `t2` )               |
|   30 | outline_db | b4369611be7ab2d27c85897632576a04bc08f50b928a1d735b62d0a140628c4c | USE INDEX |       |    1 | ind_1                                                 |    3 |        0 | SELECT * FROM `t1` WHERE `t1` . `col1` = ? AND `t1` . `col2` = ?                    |
|   31 | outline_db | 33c71541754093f78a1f2108795cfb45f8b15ec5d6bff76884f4461fb7f33419 | USE INDEX |       |    2 | ind_2                                                 |    1 |        0 | SELECT * FROM `t1` , `t2` WHERE `t1` . `col1` = `t2` . `col1` AND `t2` . `col2` = ? |
+------+------------+------------------------------------------------------------------+-----------+-------+------+-------------------------------------------------------+------+----------+-------------------------------------------------------------------------------------+
7 rows in set (0.00 sec)​

    关于HIT和OVERFLOW的说明如下。

    参数 说明
    HIT 此Statement Outline命中的次数。
    OVERFLOW 此Statement Outline没有找到Query block或相应的表的次数。
  • del_outline
    删除内存和表中的某一条Statement Outline。命令如下： 
    dbms_outln.del_outline(<Id>);

    示例：

    ​mysql> call dbms_outln.del_outline(32);
    说明 如果删除的规则不存在，系统会报相应的警告，您可以使用show warnings;查看警告内容。 
    ​mysql> call dbms_outln.del_outline(1000);
Query OK, 0 rows affected, 2 warnings (0.00 sec)

mysql> show warnings;
+---------+------+----------------------------------------------+
| Level   | Code | Message                                      |
+---------+------+----------------------------------------------+
| Warning | 7521 | Statement outline 1000 is not found in table |
| Warning | 7521 | Statement outline 1000 is not found in cache |
+---------+------+----------------------------------------------+
2 rows in set (0.00 sec)​
  • flush_outline
    如果您直接操作了表outline修改Statement Outline，您需要让Statement Outline重新生效。命令如下： 
    dbms_outln.flush_outline();

    示例：

    mysql> update mysql.outline set Position = 1 where Id = 18;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1  Changed: 1  Warnings: 0

​mysql> call dbms_outln.flush_outline(); 
Query OK, 0 rows affected (0.01 sec)​

功能测试

验证Statement Outline是否有效果，有如下两种方法：
  • 通过preview_outline进行预览。 
    mysql> call dbms_outln.preview_outline('outline_db', "select * from t1 where t1.col1 =1 and t1.col2 ='xpchild'");
+------------+------------------------------------------------------------------+------------+------------+-------+---------------------+
| SCHEMA     | DIGEST                                                           | BLOCK_TYPE | BLOCK_NAME | BLOCK | HINT                |
+------------+------------------------------------------------------------------+------------+------------+-------+---------------------+
| outline_db | b4369611be7ab2d27c85897632576a04bc08f50b928a1d735b62d0a140628c4c | TABLE      | t1         |     1 | USE INDEX (`ind_1`) |
+------------+------------------------------------------------------------------+------------+------------+-------+---------------------+
1 row in set (0.01 sec)
  • 直接使用explain查看。 
    mysql> explain select * from t1 where t1.col1 =1 and t1.col2 ='xpchild';
+----+-------------+-------+------------+------+---------------+-------+---------+-------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key   | key_len | ref   | rows | filtered | Extra       |
+----+-------------+-------+------------+------+---------------+-------+---------+-------+------+----------+-------------+
|  1 | SIMPLE      | t1    | NULL       | ref  | ind_1         | ind_1 | 5       | const |    1 |   100.00 | Using where |
+----+-------------+-------+------------+------+---------------+-------+---------+-------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

mysql> show warnings;
+-------+------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Level | Code | Message                                                                                                                                                                                                                                                 |
+-------+------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Note  | 1003 | /* select#1 */ select `outline_db`.`t1`.`id` AS `id`,`outline_db`.`t1`.`col1` AS `col1`,`outline_db`.`t1`.`col2` AS `col2` from `outline_db`.`t1` USE INDEX (`ind_1`) where ((`outline_db`.`t1`.`col1` = 1) and (`outline_db`.`t1`.`col2` = 'xpchild')) |
+-------+------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)