背景
在生产环境，MySQL 数据库实例运行过程中，一些 SQL 语句会发生执行计划的变化，导致增加了数据库稳定性的风险， 这里边有几个因素和场景，比如：随着表数据量的变化，以及统计信息的自动收集，CBO optimizer 计算得到了一个cost 更低的 plan， 又或者 表结构发生了变化，增加和删减了某些索引，或者在实例升级迁移等过程中，MySQL 自身优化器行为和算法发生了变化等。 为了能够在线应对和干预 业务 SQL 语句的执行计划， AliSQL 设计了一套利用 MySQL optimizer/index hint 来稳定执行计划的方法，称为 Statement outline，并提供了 一组管理接口方便使用（DBMS_OUTLN package）， 并在 RDS MySQL 8.0 产品上公开使用。
Outline 设计
AliSQL 8.0 outline 支持 MySQL 8.0 官方支持的所有 hint 类型，主要分为两大类：
Optimizer Hint
根据作用域（query block）和 hint 对象，又分为：Global level hint，Table/Index level hint,
Join order hint等等。
详细信息参考：https://dev.mysql.com/doc/refman/8.0/en/optimizer-hints.html
Index Hint
主要根据 index hint 的类型 （USE, FORCE, IGNORE）和 scope （FOR JOIN, FOR ORDER BY,
FOR GROUP BY）进行分类。
详细语法参考：https://dev.mysql.com/doc/refman/8.0/en/index-hints.html
为了表示和抽象这些 Hint，并能够持久化 outline，AliSQL 8.0 增加了一个系统表 mysql.outline，其结构如下：
MYSQL.OUTLINE
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'Columns
Digest/Digest_text
Outline 根据语句的特征进行匹配，这个特征就是 Digest text，根据这个 Digest Text 进行 hash 计算得到
一个 64 字节的 hash 字符串。例如：
Statement query: select * from t1 where id = 1
根据计算得到的Digest 和 Digest text 分别是：
Digest ： 36bebc61fce7e32b93926aec3fdd790dad5d895107e2d8d3848d1c60b74bcde6
Digest_text: SELECT * FROM `t1` WHERE `id` = ?
当语句 parse 完之后， 会根据 [schema + digest] 作为 hash key，进行查询匹配的 Outline。
Type
所有的 optimizer hint 的 type 统一为 OPTIMIZER.
Index hint 分为三类， 分别是：
- USE INDEX
- FORCE INDEX
- IGNORE INDEX
Scope
scope 只针对 Index hint 而言，分为四类：
- FOR GROUP BY
- FOR ORDER BY
- FOR JOIN
如果是 空串，表示 ALL
Position
Position 非常关键：
Optimizer hint 中，position 表示 Query Block， 因为所有的 optimizer hint 必须作用到 Query Block上，
这里判断比较简单， 因为 Optimizer hint 只支持 这几类关键字：
SELECT /*+ ... */ ...
INSERT /*+ ... */ ...
REPLACE /*+ ... */ ...
UPDATE /*+ ... */ ...
DELETE /*+ ... */ ...所以，position 从 1 开始，hint 作用在语句的第几个关键字锚点上，就是几。
Index hint 中， position 表示 table position， 也是从1开始，hint作用在第几个 table 锚点上，就是几。
Hint
在 Index hint 中， 这里表示的是 索引名字的列表， 比如 “ind_1, ind_2”
在 Optimizer hint 中， 这里表示的就是完整的 hint 字符串，比如：“/+ MAX_EXECUTION_TIME(1000) /”
用户接口
为了更方便的管理 Statement outline，AliSQL 设计了一个 DBMS_OUTLN package 来进行管理，并提供了 5 个native procedure 接口：
DBMS_OUTLN.add_index_outline(); 增加 index hint
DBMS_OUTLN.add_optimizer_outline(); 增加 optimizer hint
DBMS_OUTLN.preview_outline(); 预览某一个 SQL 语句命中 outline 的情况
DBMS_OUTLN.show_outline(); 展示内存中可用的所有 outline 及命中情况
DBMS_OUTLN.del_outline(); 删除内存和持久化表中的 outline
DBMS_OUTLN.flush_outline(); 刷新所有的 outline，从 mysql.outline 表中重新 load为了方便的介绍 DBMS_OUTLN 的使用，这里使用一些测试表：
CREATE TABLE `t1` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`col1` int(11) DEFAULT NULL,
`col2` varchar(100) DEFAULT NULL,
PRIMARY KEY (`id`),
KEY `ind_1` (`col1`),
KEY `ind_2` (`col2`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
CREATE TABLE `t2` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`col1` int(11) DEFAULT NULL,
`col2` varchar(100) DEFAULT NULL,
PRIMARY KEY (`id`),
KEY `ind_1` (`col1`),
KEY `ind_2` (`col2`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
ADD_INDEX_OUTLINE
语法和参数
CALL DBMS_OUTLN.add_index_outline(schema=>, digest=>, position=>, type=>,
scope=>, hint=>, query=>);
说明：
digest 和 query 可以选择其一， 如果填写了原始query语句，这个 proc 会计算 digest 和 digest text。测试 case 1
测试语句
select * from t1 where t1.col1 =1 and t1.col2 ='xpchild';
使用 ind_1 的索引
call dbms_outln.add_index_outline('outline_db', '', 1, 'USE INDEX', 'ind_1', '',
"select * from t1 where t1.col1 =1 and t1.col2 ='xpchild'");
查看 outline：
mysql> call dbms_outln.show_outline();
+------+------------+------------------------------------------------------------------+-----------+-------+------+-------+------+----------+------------------------------------------------------------------+
| ID | SCHEMA | DIGEST | TYPE | SCOPE | POS | HINT | HIT | OVERFLOW | DIGEST_TEXT |
+------+------------+------------------------------------------------------------------+-----------+-------+------+-------+------+----------+------------------------------------------------------------------+
| 30 | outline_db | b4369611be7ab2d27c85897632576a04bc08f50b928a1d735b62d0a140628c4c | USE INDEX | | 1 | ind_1 | 0 | 0 | SELECT * FROM `t1` WHERE `t1` . `col1` = ? AND `t1` . `col2` = ? |
+------+------------+------------------------------------------------------------------+-----------+-------+------+-------+------+----------+------------------------------------------------------------------+
1 row in set (0.00 sec)验证 Outline：
验证 Outline 是否其效果，可以有两种方法：
- dbms_outln.preview_outline() 进行预览
- 直接使用 explain 进行查看。
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)
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)测试 case 2
测试语句：
select * from t1, t2 where t1.col1 = t2.col1 and t2.col2 ='xpchild'测试使用 t2 表的 ind_2 索引：
call dbms_outln.add_index_outline('outline_db', '', 2, 'USE INDEX', 'ind_2', '',
"select * from t1, t2 where t1.col1 = t2.col1 and t2.col2 ='xpchild'");
验证 Outline：
mysql> explain select * from t1, t2 where t1.col1 = t2.col1 and t2.col2 ='xpchild';
+----+-------------+-------+------------+------+---------------+-------+---------+-------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+-------+---------+-------+------+----------+-------------+
| 1 | SIMPLE | t1 | NULL | ALL | ind_1 | NULL | NULL | NULL | 1 | 100.00 | NULL |
| 1 | SIMPLE | t2 | NULL | ref | ind_2 | ind_2 | 303 | const | 1 | 100.00 | Using where |
+----+-------------+-------+------------+------+---------------+-------+---------+-------+------+----------+-------------+
2 rows in set, 1 warning (0.01 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`,`outline_db`.`t2`.`id` AS `id`,`outline_db`.`t2`.`col1` AS `col1`,`outline_db`.`t2`.`col2` AS `col2` from `outline_db`.`t1` join `outline_db`.`t2` USE INDEX (`ind_2`) where ((`outline_db`.`t2`.`col1` = `outline_db`.`t1`.`col1`) and (`outline_db`.`t2`.`col2` = 'xpchild')) |
+-------+------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)ADD_OPTIMIZER_OUTLINE
语法和参数
CALL DBMS_OUTLN.add_optimizer_outline(schema=>, digest=>, query_block=>
hint=>, query=>);
说明：digest 和 query 同样可以填其一，或者都填入。proc 会自动计算digest 和 digest text。测试 case 1
增加全局 MAX_EXECUTION_TIME / SET VAR optimizer hint;
CALL DBMS_OUTLN.add_optimizer_outline("outline_db", '', 1, '/*+ MAX_EXECUTION_TIME(1000) */',
"select * from t1 where id = 1");
CALL DBMS_OUTLN.add_optimizer_outline("outline_db", '', 1, '/*+ SET_VAR(foreign_key_checks=OFF) */',
"select * from t1 where id = 1");验证 Outline：
mysql> explain select * from t1 where id = 1;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+--------------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+--------------------------------+
| 1 | SIMPLE | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | no matching row in const table |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+--------------------------------+
1 row in set, 1 warning (0.01 sec)
mysql> show warnings;
+-------+------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Level | Code | Message |
+-------+------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Note | 1003 | /* select#1 */ select /*+ MAX_EXECUTION_TIME(1000) SET_VAR(foreign_key_checks='OFF') */ NULL AS `id`,NULL AS `col1`,NULL AS `col2` from `outline_db`.`t1` where multiple equal(1, NULL) |
+-------+------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)测试 case 2
测试多表关联查询： Nested-Loop join processing
CALL DBMS_OUTLN.add_optimizer_outline('outline_db', '', 1, '/*+ BNL(t1,t2) */',
"select t1.id, t2.id from t1,t2");验证Outline：
mysql> explain select t1.id, t2.id from t1,t2;
+----+-------------+-------+------------+-------+---------------+-------+---------+------+------+----------+----------------------------------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+-------+---------+------+------+----------+----------------------------------------------------+
| 1 | SIMPLE | t1 | NULL | index | NULL | ind_1 | 5 | NULL | 1 | 100.00 | Using index |
| 1 | SIMPLE | t2 | NULL | index | NULL | ind_1 | 5 | NULL | 1 | 100.00 | Using index; Using join buffer (Block Nested Loop) |
+----+-------------+-------+------------+-------+---------------+-------+---------+------+------+----------+----------------------------------------------------+
2 rows in set, 1 warning (0.01 sec)
mysql> show warnings;
+-------+------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Level | Code | Message |
+-------+------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Note | 1003 | /* select#1 */ select /*+ BNL(`t1`@`select#1`) BNL(`t2`@`select#1`) */ `outline_db`.`t1`.`id` AS `id`,`outline_db`.`t2`.`id` AS `id` from `outline_db`.`t1` join `outline_db`.`t2` |
+-------+------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)测试 case 3
测试 subquery 中带有 query block name 的情况
CALL DBMS_OUTLN.add_optimizer_outline('outline_db', '', 2, ' /*+ QB_NAME(subq1) */',
"SELECT * FROM t1 WHERE t1.col1 IN (SELECT col1 FROM t2)");
CALL DBMS_OUTLN.add_optimizer_outline('outline_db', '', 1, '/*+ SEMIJOIN(@subq1 MATERIALIZATION, DUPSWEEDOUT) */ ',
"SELECT * FROM t1 WHERE t1.col1 IN (SELECT col1 FROM t2)");
验证 Outline：
mysql> explain SELECT * FROM t1 WHERE t1.col1 IN (SELECT col1 FROM t2);
+----+--------------+-------------+------------+--------+---------------+------------+---------+--------------------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+--------------+-------------+------------+--------+---------------+------------+---------+--------------------+------+----------+-------------+
| 1 | SIMPLE | t1 | NULL | ALL | ind_1 | NULL | NULL | NULL | 1 | 100.00 | Using where |
| 1 | SIMPLE | <subquery2> | NULL | eq_ref | <auto_key> | <auto_key> | 5 | outline_db.t1.col1 | 1 | 100.00 | NULL |
| 2 | MATERIALIZED | t2 | NULL | index | ind_1 | ind_1 | 5 | NULL | 1 | 100.00 | Using index |
+----+--------------+-------------+------------+--------+---------------+------------+---------+--------------------+------+----------+-------------+
3 rows in set, 1 warning (0.00 sec)
mysql> show warnings;
+-------+------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Level | Code | Message |
+-------+------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Note | 1003 | /* select#1 */ select /*+ SEMIJOIN(@`subq1` MATERIALIZATION, DUPSWEEDOUT) */ `outline_db`.`t1`.`id` AS `id`,`outline_db`.`t1`.`col1` AS `col1`,`outline_db`.`t1`.`col2` AS `col2` from `outline_db`.`t1` semi join (`outline_db`.`t2`) where (`<subquery2>`.`col1` = `outline_db`.`t1`.`col1`) |
+-------+------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)PREVIEW_OUTLINE
dbms_outln.preview_outline() 用于使用具体 SQL 语句，查看匹配 Outline 的情况，用于手动验证。
其语法和参数：
CALL DBMS_OUTLN.preview_outline(schema=>, query=>);例如：
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
dbms_outln.show_outline 展示 outline 在内存 cache 中 命中的情况，里边有两个字段：
HIT： 说明 这个outline 命中的次数
OVERFLOW： 说明 这个 outline hint 没有找到 query block 或者 相应的 table 的次数
例如：
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)DEL_OUTLINE
dbms_outln.del_outline() 可以删除内存和表中的某一条 outline。
语法和参数如下：
CALL DBMS_OUTLN.del_outline(outline_id=>);例如：
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
dbms_outln.flush_outline() 支持清理 cache 中 outline，并从 mysql.outline 表中重新 load。
如果用户直接修改表来加载 outline，需要调用 flush 到 cache 中。
例如：
mysql> call dbms_outln.flush_outline();
Query OK, 0 rows affected (0.01 sec)