item_id int, created_time datetime, modified_time datetime, item_name varchar(20), other varchar(20) 要求：
- 源表中有100万条数据，其中有50万created_time和item_name重复。
- 要把去重后的50万数据写入到目标表。
- 重复created_time和item_name的多条数据，可以保留任意一条，不做规则限制。
实验环境 Linux虚机：CentOS release 6.4；8G物理内存(MySQL配置4G)；100G机械硬盘；双物理CPU双核，共四个处理器；MySQL 8.0.16。
建立测试表和数据
-- 建立源表
create table t_source
(
item_id int,
created_time datetime,
modified_time datetime,
item_name varchar(20),
other varchar(20)
);
-- 建立目标表
create table t_target like t_source;
-- 生成100万测试数据，其中有50万created_time和item_name重复
delimiter //
create procedure sp_generate_data()
begin set @i := 1; while @i<=500000 do set @created_time := date_add('2017-01-01',interval @i second); set @modified_time := @created_time; set @item_name := concat('a',@i);
insert into t_source
values (@i,@created_time,@modified_time,@item_name,'other'); set @i:=@i+1;
end while;
commit; set @last_insert_id := 500000;
insert into t_source
select item_id + @last_insert_id,
created_time,
date_add(modified_time,interval @last_insert_id second),
item_name, 'other'
from t_source;
commit;
end //
delimiter ;
call sp_generate_data();
-- 源表没有主键或唯一性约束，有可能存在两条完全一样的数据，所以再插入一条记录模拟这种情况。
insert into t_source select * from t_source where item_id=1; 源表中有1000001条记录，去重后的目标表应该有500000条记录。
1. 无索引对比测试
(1)使用相关子查询
truncate t_target; insert into t_target select distinct t1.* from t_source t1 where item_id in
(select min(item_id) from t_source t2 where t1.created_time=t2.created_time and t1.item_name=t2.item_name); 这个语句很长时间都出不来结果，只看一下执行计划吧。
mysql> explain select distinct t1.* from t_source t1 where item_id in
-> (select min(item_id) from t_source t2 where t1.created_time=t2.created_time and t1.item_name=t2.item_name);
+----+--------------------+-------+------------+------+---------------+------+---------+------+--------+----------+------------------------------+| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+--------------------+-------+------------+------+---------------+------+---------+------+--------+----------+------------------------------+
| 1 | PRIMARY | t1 | NULL | ALL | NULL | NULL | NULL | NULL | 997282 | 100.00 | Using where; Using temporary || 2 | DEPENDENT SUBQUERY | t2 | NULL | ALL | NULL | NULL | NULL | NULL | 997282 | 1.00 | Using where |
+----+--------------------+-------+------------+------+---------------+------+---------+------+--------+----------+------------------------------+
2 rows in set, 3 warnings (0.00 sec) 主查询和相关子查询都是全表扫描，一共要扫描100万*100万数据行，难怪出不来结果。
(2)使用表连接
truncate t_target; insert into t_target select distinct t1.* from t_source t1,
(select min(item_id) item_id,created_time,item_name from t_source group by created_time,item_name) t2 where t1.item_id = t2.item_id; 这种方法用时14秒，查询计划如下：
mysql> explain select distinct t1.* from t_source t1, (select min(item_id) item_id,created_time,item_name from t_source group by created_time,item_name) t2 where t1.item_id = t2.item_id;
+----+-------------+------------+------------+------+---------------+-------------+---------+-----------------+--------+----------+------------------------------+| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+------------+------------+------+---------------+-------------+---------+-----------------+--------+----------+------------------------------+
| 1 | PRIMARY | t1 | NULL | ALL | NULL | NULL | NULL | NULL | 997282 | 100.00 | Using where; Using temporary || 1 | PRIMARY | | NULL | ref | 0> | | 5 | test.t1.item_id | 10 | 100.00 | Distinct |
| 2 | DERIVED | t_source | NULL | ALL | NULL | NULL | NULL | NULL | 997282 | 100.00 | Using temporary |
+----+-------------+------------+------------+------+---------------+-------------+---------+-----------------+--------+----------+------------------------------+3 rows in set, 1 warning (0.00 sec)- 内层查询扫描t_source表的100万行，建立临时表，找出去重后的最小item_id，生成导出表derived2，此导出表有50万行。
- MySQL会在导出表derived2上自动创建一个item_id字段的索引auto_key0。
- 外层查询也要扫描t_source表的100万行数据，在与导出表做链接时，对t_source表每行的item_id，使用auto_key0索引查找导出表中匹配的行，并在此时优化distinct操作，在找到第一个匹配的行后即停止查找同样值的动作。
(3)使用变量
set @a:='1000-01-01 00:00:00'; set @b:=' '; set @f:=0; truncate t_target; insert into t_target select item_id,created_time,modified_time,item_name,other from
(select t0.*,if(@a=created_time and @b=item_name,@f:=0,@f:=1) f, @a:=created_time,@b:=item_name from
(select * from t_source order by created_time,item_name) t0) t1 where f=1; 这种方法用时13秒，查询计划如下：
mysql> explain select item_id,created_time,modified_time,item_name,other
-> from
-> (select t0.*,if(@a=created_time and @b=item_name,@f:=0,@f:=1) f, @a:=created_time,@b:=item_name
-> from
-> (select * from t_source order by created_time,item_name) t0) t1 where f=1;
+----+-------------+------------+------------+------+---------------+-------------+---------+-------+--------+----------+----------------+| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+------------+------------+------+---------------+-------------+---------+-------+--------+----------+----------------+
| 1 | PRIMARY | | NULL | ref | | 0> | 4 | const | 10 | 100.00 | NULL || 2 | DERIVED | | NULL | ALL | NULL | NULL | NULL | NULL | 997282 | 100.00 | NULL |
| 3 | DERIVED | t_source | NULL | ALL | NULL | NULL | NULL | NULL | 997282 | 100.00 | Using filesort |
+----+-------------+------------+------------+------+---------------+-------------+---------+-------+--------+----------+----------------+3 rows in set, 5 warnings (0.00 sec)- 最内层的查询扫描t_source表的100万行，并使用文件排序，生成导出表derived3。
- 第二层查询要扫描derived3的100万行，生成导出表derived2，完成变量的比较和赋值，并自动创建一个导出列f上的索引auto_key0。
- 最外层使用auto_key0索引扫描derived2得到去重的结果行。
与上面方法2比较，总的扫描行数不变，都是200万行。只存在一点微小的差别，这次自动生成的索引是在常量列 f 上，而表关联自动生成的索引是在item_id列上，所以查询时间几乎相同。
至此，我们还没有在源表上创建任何索引。无论使用哪种写法，要查重都需要对created_time和item_name字段进行排序，因此很自然地想到，如果在这两个字段上建立联合索引，利用索引本身有序的特性消除额外排序，从而提高查询性能。
2. 建立created_time和item_name上的联合索引对比测试 
truncate t_target; insert into t_target select distinct t1.* from t_source t1 where item_id in
(select min(item_id) from t_source t2 where t1.created_time=t2.created_time and t1.item_name=t2.item_name); 本次用时19秒，查询计划如下：
mysql> explain select distinct t1.* from t_source t1 where item_id in
-> (select min(item_id) from t_source t2 where t1.created_time=t2.created_time and t1.item_name=t2.item_name);
+----+--------------------+-------+------------+------+---------------+----------+---------+----------------------------------------+--------+----------+------------------------------+| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+--------------------+-------+------------+------+---------------+----------+---------+----------------------------------------+--------+----------+------------------------------+
| 1 | PRIMARY | t1 | NULL | ALL | NULL | NULL | NULL | NULL | 997281 | 100.00 | Using where; Using temporary || 2 | DEPENDENT SUBQUERY | t2 | NULL | ref | idx_sort | idx_sort | 89 | test.t1.created_time,test.t1.item_name | 2 | 100.00 | Using index |
+----+--------------------+-------+------------+------+---------------+----------+---------+----------------------------------------+--------+----------+------------------------------+
2 rows in set, 3 warnings (0.00 sec)- 外层查询的t_source表是驱动表，需要扫描100万行。
- 对于驱动表每行的item_id，通过idx_sort索引查询出两行数据。
(2)使用表连接
truncate t_target; insert into t_target select distinct t1.* from t_source t1,
(select min(item_id) item_id,created_time,item_name from t_source group by created_time,item_name) t2 where t1.item_id = t2.item_id; 本次用时13秒，查询计划如下：
mysql> explain select distinct t1.* from t_source t1,
-> (select min(item_id) item_id,created_time,item_name from t_source group by created_time,item_name) t2
-> where t1.item_id = t2.item_id;
+----+-------------+------------+------------+-------+---------------+-------------+---------+-----------------+--------+----------+------------------------------+| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+------------+------------+-------+---------------+-------------+---------+-----------------+--------+----------+------------------------------+
| 1 | PRIMARY | t1 | NULL | ALL | NULL | NULL | NULL | NULL | 997281 | 100.00 | Using where; Using temporary || 1 | PRIMARY | | NULL | ref | 0> | | 5 | test.t1.item_id | 10 | 100.00 | Distinct |
| 2 | DERIVED | t_source | NULL | index | idx_sort | idx_sort | 94 | NULL | 997281 | 100.00 | Using index |
+----+-------------+------------+------------+-------+---------------+-------------+---------+-----------------+--------+----------+------------------------------+3 rows in set, 1 warning (0.00 sec) 和没有索引相比，子查询虽然从全表扫描变为了全索引扫描，但还是需要扫描100万行记录。因此查询性能提升并不是明显。
(3)使用变量
set @a:='1000-01-01 00:00:00'; set @b:=' '; set @f:=0; truncate t_target; insert into t_target select item_id,created_time,modified_time,item_name,other from
(select t0.*,if(@a=created_time and @b=item_name,@f:=0,@f:=1) f, @a:=created_time,@b:=item_name from
(select * from t_source order by created_time,item_name) t0) t1 where f=1; 本次用时13秒，查询计划与没有索引时的完全相同。可见索引对这种写法没有作用。能不能消除嵌套，只用一层查询出结果呢？
(4)使用变量，并且消除嵌套查询
set @a:='1000-01-01 00:00:00'; set @b:=' '; truncate t_target; insert into t_target select * from t_source force index (idx_sort) where (@a!=created_time or @b!=item_name) and (@a:=created_time) is not null and (@b:=item_name) is not null order by created_time,item_name; 本次用时12秒，查询计划如下： 本次用时12秒，查询计划如下：
mysql> explain select * from t_source force index (idx_sort)
-> where (@a!=created_time or @b!=item_name) and (@a:=created_time) is not null and (@b:=item_name) is not null
-> order by created_time,item_name;
+----+-------------+----------+------------+-------+---------------+----------+---------+------+--------+----------+-------------+| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+----------+------------+-------+---------------+----------+---------+------+--------+----------+-------------+
| 1 | SIMPLE | t_source | NULL | index | NULL | idx_sort | 94 | NULL | 997281 | 99.00 | Using where |
+----+-------------+----------+------------+-------+---------------+----------+---------+------+--------+----------+-------------+1 row in set, 3 warnings (0.00 sec) 该语句具有以下特点：
- 消除了嵌套子查询，只需要对t_source表进行一次全索引扫描，查询计划已达最优。
- 无需distinct二次查重。
- 变量判断与赋值只出现在where子句中。
- 利用索引消除了filesort。
在MySQL 8之前，该语句是单线程去重的最佳解决方案。仔细分析这条语句，发现它巧妙地利用了SQL语句的逻辑查询处理步骤和索引特性。一条SQL查询的逻辑步骤为：
- 执行笛卡尔乘积(交叉连接)
- 应用ON筛选器(连接条件)
- 添加外部行(outer join)
- 应用where筛选器
- 分组
- 应用cube或rollup
- 应用having筛选器
- 处理select列表
- 应用distinct子句
- 应用order by子句
- 应用limit子句
每条查询语句的逻辑执行步骤都是这11步的子集。拿这条查询语句来说，其执行顺序为：强制通过索引idx_sort查找数据行 -> 应用where筛选器 -> 处理select列表 -> 应用order by子句。
为了使变量能够按照created_time和item_name的排序顺序进行赋值和比较，必须按照索引顺序查找数据行。这里的force index (idx_sort)提示就起到了这个作用，必须这样写才能使整条查重语句成立。否则，因为先扫描表才处理排序，因此不能保证变量赋值的顺序，也就不能确保查询结果的正确性。order by子句同样不可忽略，否则即使有force index提示，MySQL也会使用全表扫描而不是全索引扫描，从而使结果错误。索引同时保证了created_time,item_name的顺序，避免了文件排序。force index (idx_sort)提示和order by子句缺一不可，索引idx_sort在这里可谓恰到好处、一举两得。
查询语句开始前，先给变量初始化为数据中不可能出现的值，然后进入where子句从左向右判断。先比较变量和字段的值，再将本行created_time和item_name的值赋给变量，按created_time、item_name的顺序逐行处理。item_name是字符串类型，(@b:=item_name)不是有效的布尔表达式，因此要写成(@b:=item_name) is not null。
最后补充一句，这里忽略了“insert into t_target select * from t_source group by created_time,item_name;”的写法，因为它受“sql_mode='ONLY_FULL_GROUP_BY'”的限制。
MySQL 8中新增的窗口函数使得原来麻烦的去重操作变得很简单。
truncate t_target; insert into t_target select item_id, created_time, modified_time, item_name, otherfrom (select *, row_number() over(partition by created_time,item_name) as rnfrom t_source) t where rn=1; 这个语句执行只需要12秒，而且写法清晰易懂，其查询计划如下：
mysql> explain select item_id, created_time, modified_time, item_name, other
-> from (select *, row_number() over(partition by created_time,item_name) as rn
-> from t_source) t where rn=1;
+----+-------------+------------+------------+------+---------------+-------------+---------+-------+--------+----------+----------------+| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+------------+------------+------+---------------+-------------+---------+-------+--------+----------+----------------+
| 1 | PRIMARY | | NULL | ref | | 0> | 8 | const | 10 | 100.00 | NULL || 2 | DERIVED | t_source | NULL | ALL | NULL | NULL | NULL | NULL | 997281 | 100.00 | Using filesort |
+----+-------------+------------+------------+------+---------------+-------------+---------+-------+--------+----------+----------------+
2 rows in set, 2 warnings (0.00 sec) 该查询对t_source表进行了一次全表扫描，同时用filesort对表按分区字段created_time、item_name进行了排序。外层查询从每个分区中保留一条数据。因为重复created_time和item_name的多条数据中可以保留任意一条，所以oevr中不需要使用order by子句。
从执行计划看，窗口函数去重语句似乎没有消除嵌套查询的变量去重好，但此方法实际执行是最快的。
MySQL窗口函数说明参见
“https://dev.mysql.com/doc/refman/8.0/en/window-functions.html”
前面已经将单条查重语句调整到最优，但还是以单线程方式执行。能否利用多处理器，让去重操作多线程并行执行，从而进一步提高速度呢？比如我的实验环境是4处理器，如果使用4个线程同时执行查重SQL，理论上应该接近4倍的性能提升。 1. 数据分片
在生成测试数据时，created_time采用每条记录加一秒的方式，也就是最大和在最小的时间差为50万秒，而且数据均匀分布，因此先把数据平均分成4份。 (1)查询出4份数据的created_time边界值
mysql> select date_add('2017-01-01',interval 125000 second) dt1,
-> date_add('2017-01-01',interval 2*125000 second) dt2,
-> date_add('2017-01-01',interval 3*125000 second) dt3,
-> max(created_time) dt4
-> from t_source;
+---------------------+---------------------+---------------------+---------------------+| dt1 | dt2 | dt3 | dt4 |
+---------------------+---------------------+---------------------+---------------------+
| 2017-01-02 10:43:20 | 2017-01-03 21:26:40 | 2017-01-05 08:10:00 | 2017-01-06 18:53:20 |
+---------------------+---------------------+---------------------+---------------------+1 row in set (0.00 sec)(2)查看每份数据的记录数，确认数据平均分布
4份数据的并集应该覆盖整个源数据集，并且数据之间是不重复的。也就是说4份数据的created_time要连续且互斥，连续保证处理全部数据，互斥确保了不需要二次查重。实际上这和时间范围分区的概念类似，或许用分区表更好些，只是这里省略了重建表的步骤。
2. 建立查重的存储过程 有了以上信息我们就可以写出4条语句处理全部数据。为了调用接口尽量简单，建立下面的存储过程。
delimiter //
create procedure sp_unique(i smallint) begin set @a:='1000-01-01 00:00:00'; set @b:=' '; if (i<4) then
insert into t_target
select * from t_source force index (idx_sort)
where created_time >= date_add('2017-01-01',interval (i-1)*125000 second) and created_time '2017-01-01',interval i*125000 second) and (@a!=created_time or @b!=item_name) and (@a:=created_time) is not null and (@b:=item_name) is not null
order by created_time,item_name; else
insert into t_target
select * from t_source force index (idx_sort)
where created_time >= date_add('2017-01-01',interval (i-1)*125000 second) and created_time <= date_add('2017-01-01',interval i*125000 second) and (@a!=created_time or @b!=item_name) and (@a:=created_time) is not null and (@b:=item_name) is not null
order by created_time,item_name; end if; end // 查询语句的执行计划如下：
mysql> explain select * from t_source force index (idx_sort)
-> where created_time >= date_add('2017-01-01',interval (1-1)*125000 second)
-> and created_time '2017-01-01',interval 1*125000 second)
-> and (@a!=created_time or @b!=item_name)
-> and (@a:=created_time) is not null
-> and (@b:=item_name) is not null
-> order by created_time,item_name;
+----+-------------+----------+------------+-------+---------------+----------+---------+------+--------+----------+-----------------------+| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+----------+------------+-------+---------------+----------+---------+------+--------+----------+-----------------------+
| 1 | SIMPLE | t_source | NULL | range | idx_sort | idx_sort | 6 | NULL | 498640 | 100.00 | Using index condition |
+----+-------------+----------+------------+-------+---------------+----------+---------+------+--------+----------+-----------------------+1 row in set, 3 warnings (0.00 sec) MySQL优化器进行索引范围扫描，并且使用索引条件下推(ICP)优化查询。
3. 并行执行
下面分别使用shell后台进程和MySQL Schedule Event实现并行。 (1)shell后台进程 建立duplicate_removal.sh文件，内容如下：
执行脚本文件
./duplicate_removal.sh 执行输出如下：
[mysql@hdp2~]$./duplicate_removal.sh14:27:3014:27:35 这种方法用时5秒，并行执行的4个过程调用分别用时为4.87秒、4.88秒、4.91秒、4.73秒：
[mysql@hdp2~]$cat par_sql1_1.log | sed '/^$/d'
mysql: [Warning] Using a password on the command line interface can be insecure.--------------call sp_unique(1)--------------Query OK, 124999 rows affected (4.87 sec)
Bye
[mysql@hdp2~]$cat par_sql1_2.log | sed '/^$/d'
mysql: [Warning] Using a password on the command line interface can be insecure.--------------call sp_unique(2)--------------Query OK, 125000 rows affected (4.88 sec)
Bye
[mysql@hdp2~]$cat par_sql1_3.log | sed '/^$/d'
mysql: [Warning] Using a password on the command line interface can be insecure.--------------call sp_unique(3)--------------Query OK, 125000 rows affected (4.91 sec)
Bye
[mysql@hdp2~]$cat par_sql1_4.log | sed '/^$/d'
mysql: [Warning] Using a password on the command line interface can be insecure.--------------call sp_unique(4)--------------Query OK, 125001 rows affected (4.73 sec)
Bye
[mysql@hdp2~]$ 可以看到，每个过程的执行时间均4.85，因为是并行执行，总的过程执行时间为最慢的4.91秒，比单线程速度提高了2.5倍。
(2)MySQL Schedule Event
- 建立事件历史日志表
-- 用于查看事件执行时间等信息create table t_event_history (
dbname varchar(128) not null default '',
eventname varchar(128) not null default '',
starttime datetime(3) not null default '1000-01-01 00:00:00',
endtime datetime(3) default null,
issuccess int(11) default null, duration int(11) default null,
errormessage varchar(512) default null,
randno int(11) default null
);- 为每个并发线程创建一个事件
delimiter //create event ev1 on schedule at current_timestamp + interval 1 hour on completion preserve disable do begindeclare r_code char(5) default '00000'; declare r_msg text; declare v_error integer; declare v_starttime datetime default now(3); declare v_randno integer default floor(rand()*100001); insert into t_event_history (dbname,eventname,starttime,randno) #作业名 values(database(),'ev1', v_starttime,v_randno); begin #异常处理段 declare continue handler for sqlexception begin set v_error = 1;
get diagnostics condition 1 r_code = returned_sqlstate , r_msg = message_text; end; #此处为实际调用的用户程序过程 call sp_unique(1); end; update t_event_history set endtime=now(3),issuccess=isnull(v_error),duration=timestampdiff(microsecond,starttime,now(3)), errormessage=concat('error=',r_code,', message=',r_msg),randno=null where starttime=v_starttime and randno=v_randno; end
// create event ev2 on schedule at current_timestamp + interval 1 hour on completion preserve disable do begindeclare r_code char(5) default '00000'; declare r_msg text; declare v_error integer; declare v_starttime datetime default now(3); declare v_randno integer default floor(rand()*100001); insert into t_event_history (dbname,eventname,starttime,randno) #作业名 values(database(),'ev2', v_starttime,v_randno); begin #异常处理段 declare continue handler for sqlexception begin set v_error = 1;
get diagnostics condition 1 r_code = returned_sqlstate , r_msg = message_text; end; #此处为实际调用的用户程序过程 call sp_unique(2); end; update t_event_history set endtime=now(3),issuccess=isnull(v_error),duration=timestampdiff(microsecond,starttime,now(3)), errormessage=concat('error=',r_code,', message=',r_msg),randno=null where starttime=v_starttime and randno=v_randno; end
// create event ev3 on schedule at current_timestamp + interval 1 hour on completion preserve disable do begindeclare r_code char(5) default '00000'; declare r_msg text; declare v_error integer; declare v_starttime datetime default now(3); declare v_randno integer default floor(rand()*100001); insert into t_event_history (dbname,eventname,starttime,randno) #作业名 values(database(),'ev3', v_starttime,v_randno); begin #异常处理段 declare continue handler for sqlexception begin set v_error = 1;
get diagnostics condition 1 r_code = returned_sqlstate , r_msg = message_text; end; #此处为实际调用的用户程序过程 call sp_unique(3); end; update t_event_history set endtime=now(3),issuccess=isnull(v_error),duration=timestampdiff(microsecond,starttime,now(3)), errormessage=concat('error=',r_code,', message=',r_msg),randno=null where starttime=v_starttime and randno=v_randno; end
// create event ev4 on schedule at current_timestamp + interval 1 hour on completion preserve disable do begindeclare r_code char(5) default '00000'; declare r_msg text; declare v_error integer; declare v_starttime datetime default now(3); declare v_randno integer default floor(rand()*100001); insert into t_event_history (dbname,eventname,starttime,randno) #作业名 values(database(),'ev4', v_starttime,v_randno); begin #异常处理段 declare continue handler for sqlexception begin set v_error = 1;
get diagnostics condition 1 r_code = returned_sqlstate , r_msg = message_text; end; #此处为实际调用的用户程序过程 call sp_unique(4); end; update t_event_history set endtime=now(3),issuccess=isnull(v_error),duration=timestampdiff(microsecond,starttime,now(3)), errormessage=concat('error=',r_code,', message=',r_msg),randno=null where starttime=v_starttime and randno=v_randno; end
// 为了记录每个事件执行的时间，在事件定义中增加了操作日志表的逻辑，因为每个事件中只多执行了一条insert，一条update，4个事件总共多执行8条很简单的语句，对测试的影响可以忽略不计。执行时间精确到毫秒。
- 触发事件执行
mysql -vvv -u root -p123456 test -e "truncate t_target;alter event ev1 on schedule at current_timestamp enable;alter event ev2 on schedule at current_timestamp enable;alter event ev3 on schedule at current_timestamp enable;alter event ev4 on schedule at current_timestamp enable;" 该命令行顺序触发了4个事件，但不会等前一个执行完才执行下一个，而是立即向下执行。这可从命令的输出可以清除看到：
[mysql@hdp2~]$mysql -vvv -u root -p123456 test -e "truncate t_target;alter event ev1 on schedule at current_timestamp enable;alter event ev2 on schedule at current_timestamp enable;alter event ev3 on schedule at current_timestamp enable;alter event ev4 on schedule at current_timestamp enable;"
mysql: [Warning] Using a password on the command line interface can be insecure.--------------truncate t_target--------------Query OK, 0 rows affected (0.06 sec)--------------alter event ev1 on schedule at current_timestamp enable--------------Query OK, 0 rows affected (0.02 sec)--------------alter event ev2 on schedule at current_timestamp enable--------------Query OK, 0 rows affected (0.00 sec)--------------alter event ev3 on schedule at current_timestamp enable--------------Query OK, 0 rows affected (0.02 sec)--------------alter event ev4 on schedule at current_timestamp enable--------------Query OK, 0 rows affected (0.00 sec)
Bye
[mysql@hdp2~]$- 查看事件执行日志
mysql> select * from test.t_event_history;
+--------+-----------+-------------------------+-------------------------+-----------+----------+--------------+--------+| dbname | eventname | starttime | endtime | issuccess | duration | errormessage | randno |
+--------+-----------+-------------------------+-------------------------+-----------+----------+--------------+--------+
| test | ev1 | 2019-07-31 14:38:04.000 | 2019-07-31 14:38:09.389 | 1 | 5389000 | NULL | NULL || test | ev2 | 2019-07-31 14:38:04.000 | 2019-07-31 14:38:09.344 | 1 | 5344000 | NULL | NULL |
| test | ev3 | 2019-07-31 14:38:05.000 | 2019-07-31 14:38:09.230 | 1 | 4230000 | NULL | NULL || test | ev4 | 2019-07-31 14:38:05.000 | 2019-07-31 14:38:09.344 | 1 | 4344000 | NULL | NULL |
+--------+-----------+-------------------------+-------------------------+-----------+----------+--------------+--------+
4 rows in set (0.00 sec) 可以看到，每个过程的执行均为4.83秒，又因为是并行执行的，因此总的执行之间为最慢的5.3秒，优化效果和shell后台进程方式几乎相同。
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