生產(chǎn)環(huán)境中，SQL語(yǔ)句的執(zhí)行計(jì)劃經(jīng)常會(huì)發(fā)生改變，導(dǎo)致數(shù)據(jù)庫(kù)不穩(wěn)定。阿里云利用Optimizer Hint和Index Hint讓MySQL穩(wěn)定執(zhí)行計(jì)劃，該方法稱為Statement Outline，并提供了工具包（DBMS_OUTLN）便于您快捷使用。
前提條件
RDS實(shí)例版本如下：
- MySQL 8.0
- MySQL 5.7
功能設(shè)計(jì)
Statement Outline支持官方MySQL 8.0、MySQL 5.7的所有hint類型，分為如下兩類：
- Optimizer Hint
根據(jù)作用域和hint對(duì)象，分為Global level hint、Table/Index level hint、Join order hint等。詳情請(qǐng)參見MySQL官網(wǎng)。
- Index Hint
根據(jù)Index Hint的類型和范圍進(jìn)行分類。詳情請(qǐng)參見MySQL官網(wǎng)
Statement Outline表介紹
AliSQL內(nèi)置了一個(gè)系統(tǒng)表（outline）保存hint，系統(tǒng)啟動(dòng)時(shí)會(huì)自動(dòng)創(chuàng)建該表，無(wú)需您手動(dòng)創(chuàng)建。這里提供表的創(chuàng)建語(yǔ)句供您參考：
?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'?參數(shù)說(shuō)明如下。
| 參數(shù) | 說(shuō)明 |
|---|---|
| Id | Outline ID。 |
| Schema_name | 數(shù)據(jù)庫(kù)名。 |
| Digest | Digest_text進(jìn)行hash計(jì)算得到的64字節(jié)的hash字符串。 |
| Digest_text | SQL語(yǔ)句的特征。 |
| Type |
|
| Scope | 僅Index Hint需要提供，分為如下三類：
空串表示所有類型的Index Hint。 |
| State | 本規(guī)則是否啟用。 |
| Position |
|
| Hint |
|
管理Statement Outline
為了便捷地管理Statement Outline，AliSQL在DBMS_OUTLN中定義了六個(gè)本地存儲(chǔ)規(guī)則。詳細(xì)說(shuō)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>');說(shuō)明 Digest和Query（原始SQL語(yǔ)句）可以任選其一。如果填寫Query，DBMS_OUTLN會(huì)計(jì)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>');說(shuō)明 Digest和Query（原始SQL語(yǔ)句）可以任選其一。如果填寫Query，DBMS_OUTLN會(huì)計(jì)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的情況，可用于手動(dòng)驗(yàn)證。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在內(nèi)存中命中的情況。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)?關(guān)于HIT和OVERFLOW的說(shuō)明如下。
參數(shù) 說(shuō)明 HIT 此Statement Outline命中的次數(shù)。 OVERFLOW 此Statement Outline沒有找到Query block或相應(yīng)的表的次數(shù)。 - del_outline
刪除內(nèi)存和表中的某一條Statement Outline。命令如下：
dbms_outln.del_outline(<Id>);示例：
?mysql> call dbms_outln.del_outline(32);說(shuō)明 如果刪除的規(guī)則不存在，系統(tǒng)會(huì)報(bào)相應(yīng)的警告，您可以使用show warnings;查看警告內(nèi)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，您需要讓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)?
功能測(cè)試
驗(yàn)證Statement Outline是否有效果，有如下兩種方法：
- 通過(guò)preview_outline進(jìn)行預(yù)覽。
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)