背景
在生産環境，MySQL 資料庫執行個體運作過程中，一些 SQL 語句會發生執行計劃的變化，導緻增加了資料庫穩定性的風險， 這裡邊有幾個因素和場景，比如：随着表資料量的變化，以及統計資訊的自動收集，CBO optimizer 計算得到了一個cost 更低的 plan， 又或者 表結構發生了變化，增加和删減了某些索引，或者在執行個體更新遷移等過程中，MySQL 自身優化器行為和算法發生了變化等。 為了能夠線上應對和幹預 業務 SQL 語句的執行計劃， AliSQL 設計了一套利用 MySQL optimizer/index hint 來穩定執行計劃的方法，稱為 Statement outline，并提供了 一組管理接口友善使用（DBMS_OUTLN package）， 并在 RDS MySQL 8.0 産品上公開使用。
Outline 設計
AliSQL 8.0 outline 支援 MySQL 8.0 官方支援的所有 hint 類型，主要分為兩大類：
Optimizer Hint
根據作用域（query block）和 hint 對象，又分為：Global level hint，Table/Index level hint,
Join order hint等等。
詳細資訊參考：
https://dev.mysql.com/doc/refman/8.0/en/optimizer-hints.htmlIndex Hint
主要根據 index hint 的類型 （USE, FORCE, IGNORE）和 scope （FOR JOIN, FOR ORDER BY,
FOR GROUP BY）進行分類。
詳細文法參考：
https://dev.mysql.com/doc/refman/8.0/en/index-hints.html為了表示和抽象這些 Hint，并能夠持久化 outline，AliSQL 8.0 增加了一個系統表 mysql.outline，其結構如下：
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 根據語句的特征進行比對，這個特征就是 Digest text，根據這個 Digest Text 進行 hash 計算得到
一個 64 位元組的 hash 字元串。例如：
Statement query: select * from t1 where id = 1
根據計算得到的Digest 和 Digest text 分别是：
Digest ： 36bebc61fce7e32b93926aec3fdd790dad5d895107e2d8d3848d1c60b74bcde6
Digest_text: SELECT * FROM `t1` WHERE `id` = ? 當語句 parse 完之後， 會根據 [schema + digest] 作為 hash key，進行查詢比對的 Outline。
Type
所有的 optimizer hint 的 type 統一為 OPTIMIZER.
Index hint 分為三類， 分别是：
- USE INDEX
- FORCE INDEX
- IGNORE INDEX
Scope
scope 隻針對 Index hint 而言，分為四類：
- FOR GROUP BY
- FOR ORDER BY
- FOR JOIN
如果是 空串，表示 ALL
Position
Position 非常關鍵：
Optimizer hint 中，position 表示 Query Block， 因為所有的 optimizer hint 必須作用到 Query Block上，
這裡判斷比較簡單， 因為 Optimizer hint 隻支援 這幾類關鍵字：
SELECT /*+ ... */ ...
INSERT /*+ ... */ ...
REPLACE /*+ ... */ ...
UPDATE /*+ ... */ ...
DELETE /*+ ... */ ... 是以，position 從 1 開始，hint 作用在語句的第幾個關鍵字錨點上，就是幾。
Index hint 中， position 表示 table position， 也是從1開始，hint作用在第幾個 table 錨點上，就是幾。
Hint
在 Index hint 中， 這裡表示的是 索引名字的清單， 比如 “ind_1, ind_2”
在 Optimizer hint 中， 這裡表示的就是完整的 hint 字元串，比如：“/+ MAX_EXECUTION_TIME(1000) /”
使用者接口
為了更友善的管理 Statement outline，AliSQL 設計了一個 DBMS_OUTLN package 來進行管理，并提供了 5 個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(); 預覽某一個 SQL 語句命中 outline 的情況
DBMS_OUTLN.show_outline(); 展示記憶體中可用的所有 outline 及命中情況
DBMS_OUTLN.del_outline(); 删除記憶體和持久化表中的 outline
DBMS_OUTLN.flush_outline(); 重新整理所有的 outline，從 mysql.outline 表中重新 load 為了友善的介紹 DBMS_OUTLN 的使用，這裡使用一些測試表：
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
文法和參數
CALL DBMS_OUTLN.add_index_outline(schema=>, digest=>, position=>, type=>,
scope=>, hint=>, query=>);
說明：
digest 和 query 可以選擇其一， 如果填寫了原始query語句，這個 proc 會計算 digest 和 digest text。 測試 case 1
測試語句
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'");
檢視 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) 驗證 Outline：
驗證 Outline 是否其效果，可以有兩種方法：
- dbms_outln.preview_outline() 進行預覽
- 直接使用 explain 進行檢視。
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) 測試 case 2
測試語句：
select * from t1, t2 where t1.col1 = t2.col1 and t2.col2 ='xpchild' 測試使用 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'");
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
CALL DBMS_OUTLN.add_optimizer_outline(schema=>, digest=>, query_block=>
hint=>, query=>);
說明：digest 和 query 同樣可以填其一，或者都填入。proc 會自動計算digest 和 digest text。 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"); 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) 測試多表關聯查詢： Nested-Loop join processing
CALL DBMS_OUTLN.add_optimizer_outline('outline_db', '', 1, '/*+ BNL(t1,t2) */',
"select t1.id, t2.id from t1,t2"); 驗證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) 測試 case 3
測試 subquery 中帶有 query block name 的情況
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)");
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() 用于使用具體 SQL 語句，檢視比對 Outline 的情況，用于手動驗證。
其文法和參數：
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 在記憶體 cache 中 命中的情況，裡邊有兩個字段：
HIT： 說明 這個outline 命中的次數
OVERFLOW： 說明 這個 outline hint 沒有找到 query block 或者 相應的 table 的次數
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() 可以删除記憶體和表中的某一條 outline。
文法和參數如下：
CALL DBMS_OUTLN.del_outline(outline_id=>); 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() 支援清理 cache 中 outline，并從 mysql.outline 表中重新 load。
如果使用者直接修改表來加載 outline，需要調用 flush 到 cache 中。
mysql> call dbms_outln.flush_outline();
Query OK, 0 rows affected (0.01 sec) ![超幹貨連載（一）RDS for MySQL 三節點企業版 Learner 隻讀執行個體解析[圖]](data:image/gif;base64,R0lGODlhAQABAIAAAP///wAAACwAAAAAAQABAAACAkQBADs=)