数据库内核月报 - 2019 / 07

MySQL · 最佳实践 · Statement Outline

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:

1 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

2 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

1 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。

2 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

3  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

4 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。

5 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=>,
      hint=>, scope=>,  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:

  1. dbms_outln.preview_outline() 进行预览
  2. 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)