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將MySQL去重操作優(yōu)化到極致的操作方法

1.源表中有100萬(wàn)條數(shù)據(jù)，其中有50萬(wàn)created_time和item_name重復(fù)。
2.要把去重后的50萬(wàn)數(shù)據(jù)寫(xiě)入到目標(biāo)表。
3.重復(fù)created_time和item_name的多條數(shù)據(jù)，可以保留任意一條，不做規(guī)則限制。

•實(shí)驗(yàn)環(huán)境

Linux虛機(jī)：CentOS release 6.4；8G物理內(nèi)存（MySQL配置4G）；100G機(jī)械硬盤(pán)；雙物理CPU雙核，共四個(gè)處理器；MySQL 8.0.16。

•建立測(cè)試表和數(shù)據(jù)

-- 建立源表
create table t_source 
( item_id int, 
 created_time datetime, 
 modified_time datetime, 
 item_name varchar(20), 
 other varchar(20) 
); 
-- 建立目標(biāo)表
create table t_target like t_source; 
-- 生成100萬(wàn)測(cè)試數(shù)據(jù)，其中有50萬(wàn)created_time和item_name重復(fù)
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(); 

-- 源表沒(méi)有主鍵或唯一性約束，有可能存在兩條完全一樣的數(shù)據(jù)，所以再插入一條記錄模擬這種情況。
insert into t_source select * from t_source where item_id=1;

 源表中有1000001條記錄，去重后的目標(biāo)表應(yīng)該有500000條記錄。
mysql> select count(*),count(distinct created_time,item_name) from t_source;
+----------+----------------------------------------+
| count(*) | count(distinct created_time,item_name) |
+----------+----------------------------------------+
| 1000001 |   500000 |
+----------+----------------------------------------+
1 row in set (1.92 sec)

一、巧用索引與變量

1. 無(wú)索引對(duì)比測(cè)試

（1）使用相關(guān)子查詢

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);

這個(gè)語(yǔ)句很長(zhǎng)時(shí)間都出不來(lái)結(jié)果，只看一下執(zhí)行計(jì)劃吧。

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)

主查詢和相關(guān)子查詢都是全表掃描，一共要掃描100萬(wàn)*100萬(wàn)數(shù)據(jù)行，難怪出不來(lái)結(jié)果。

（2）使用表連接

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;

這種方法用時(shí)14秒，查詢計(jì)劃如下：

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 | derived2> | NULL | ref | auto_key0> | auto_key0> | 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)

•內(nèi)層查詢掃描t_source表的100萬(wàn)行，建立臨時(shí)表，找出去重后的最小item_id，生成導(dǎo)出表derived2，此導(dǎo)出表有50萬(wàn)行。
•MySQL會(huì)在導(dǎo)出表derived2上自動(dòng)創(chuàng)建一個(gè)item_id字段的索引auto_key0。
•外層查詢也要掃描t_source表的100萬(wàn)行數(shù)據(jù)，在與導(dǎo)出表做鏈接時(shí)，對(duì)t_source表每行的item_id，使用auto_key0索引查找導(dǎo)出表中匹配的行，并在此時(shí)優(yōu)化distinct操作，在找到第一個(gè)匹配的行后即停止查找同樣值的動(dòng)作。

（3）使用變量

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;

這種方法用時(shí)13秒，查詢計(jì)劃如下：

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 | derived2> | NULL | ref | auto_key0> | auto_key0> | 4 | const | 10 | 100.00 | NULL |
| 2 | DERIVED | derived3> | 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)

•最內(nèi)層的查詢掃描t_source表的100萬(wàn)行，并使用文件排序，生成導(dǎo)出表derived3。
•第二層查詢要掃描derived3的100萬(wàn)行，生成導(dǎo)出表derived2，完成變量的比較和賦值，并自動(dòng)創(chuàng)建一個(gè)導(dǎo)出列f上的索引auto_key0。
•最外層使用auto_key0索引掃描derived2得到去重的結(jié)果行。

與上面方法2比較，總的掃描行數(shù)不變，都是200萬(wàn)行。只存在一點(diǎn)微小的差別，這次自動(dòng)生成的索引是在常量列 f 上，而表關(guān)聯(lián)自動(dòng)生成的索引是在item_id列上，所以查詢時(shí)間幾乎相同。

至此，我們還沒(méi)有在源表上創(chuàng)建任何索引。無(wú)論使用哪種寫(xiě)法，要查重都需要對(duì)created_time和item_name字段進(jìn)行排序，因此很自然地想到，如果在這兩個(gè)字段上建立聯(lián)合索引，利用索引本身有序的特性消除額外排序，從而提高查詢性能。

2. 建立created_time和item_name上的聯(lián)合索引對(duì)比測(cè)試

-- 建立created_time和item_name字段的聯(lián)合索引
create index idx_sort on t_source(created_time,item_name,item_id); 
analyze table t_source;

（1）使用相關(guān)子查詢

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);

本次用時(shí)19秒，查詢計(jì)劃如下：

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)

•外層查詢的t_source表是驅(qū)動(dòng)表，需要掃描100萬(wàn)行。

•對(duì)于驅(qū)動(dòng)表每行的item_id，通過(guò)idx_sort索引查詢出兩行數(shù)據(jù)。

（2）使用表連接

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;

本次用時(shí)13秒，查詢計(jì)劃如下：

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 | derived2> | NULL | ref | auto_key0> | auto_key0> | 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)

和沒(méi)有索引相比，子查詢雖然從全表掃描變?yōu)榱巳饕龗呙?，但還是需要掃描100萬(wàn)行記錄。因此查詢性能提升并不是明顯。

（3）使用變量

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;

本次用時(shí)13秒，查詢計(jì)劃與沒(méi)有索引時(shí)的完全相同?？梢?jiàn)索引對(duì)這種寫(xiě)法沒(méi)有作用。能不能消除嵌套，只用一層查詢出結(jié)果呢？

（4）使用變量，并且消除嵌套查詢

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;

本次用時(shí)12秒，查詢計(jì)劃如下：

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)

該語(yǔ)句具有以下特點(diǎn)：

•消除了嵌套子查詢，只需要對(duì)t_source表進(jìn)行一次全索引掃描，查詢計(jì)劃已達(dá)最優(yōu)。
•無(wú)需distinct二次查重。
•變量判斷與賦值只出現(xiàn)在where子句中。
•利用索引消除了filesort。

在MySQL 8之前，該語(yǔ)句是單線程去重的最佳解決方案。仔細(xì)分析這條語(yǔ)句，發(fā)現(xiàn)它巧妙地利用了SQL語(yǔ)句的邏輯查詢處理步驟和索引特性。一條SQL查詢的邏輯步驟為：

1.執(zhí)行笛卡爾乘積（交叉連接）
2.應(yīng)用ON篩選器（連接條件）
3.添加外部行（outer join）
4.應(yīng)用where篩選器
5.分組
6.應(yīng)用cube或rollup
7.應(yīng)用having篩選器
8.處理select列表
9.應(yīng)用distinct子句
10.應(yīng)用order by子句
11.應(yīng)用limit子句

每條查詢語(yǔ)句的邏輯執(zhí)行步驟都是這11步的子集。拿這條查詢語(yǔ)句來(lái)說(shuō)，其執(zhí)行順序?yàn)椋簭?qiáng)制通過(guò)索引idx_sort查找數(shù)據(jù)行 -> 應(yīng)用where篩選器 -> 處理select列表 -> 應(yīng)用order by子句。

為了使變量能夠按照created_time和item_name的排序順序進(jìn)行賦值和比較，必須按照索引順序查找數(shù)據(jù)行。這里的force index (idx_sort)提示就起到了這個(gè)作用，必須這樣寫(xiě)才能使整條查重語(yǔ)句成立。否則，因?yàn)橄葤呙璞聿盘幚砼判?，因此不能保證變量賦值的順序，也就不能確保查詢結(jié)果的正確性。order by子句同樣不可忽略，否則即使有force index提示，MySQL也會(huì)使用全表掃描而不是全索引掃描，從而使結(jié)果錯(cuò)誤。索引同時(shí)保證了created_time,item_name的順序，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在這里可謂恰到好處、一舉兩得。

查詢語(yǔ)句開(kāi)始前，先給變量初始化為數(shù)據(jù)中不可能出現(xiàn)的值，然后進(jìn)入where子句從左向右判斷。先比較變量和字段的值，再將本行created_time和item_name的值賦給變量，按created_time、item_name的順序逐行處理。item_name是字符串類型，(@b:=item_name)不是有效的布爾表達(dá)式，因此要寫(xiě)成(@b:=item_name) is not null。

最后補(bǔ)充一句，這里忽略了“insert into t_target select * from t_source group by created_time,item_name;”的寫(xiě)法，因?yàn)樗?code>“sql_mode='ONLY_FULL_GROUP_BY'”的限制。

二、利用窗口函數(shù)

MySQL 8中新增的窗口函數(shù)使得原來(lái)麻煩的去重操作變得很簡(jiǎn)單。

truncate t_target; 
insert into t_target 
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;

這個(gè)語(yǔ)句執(zhí)行只需要12秒，而且寫(xiě)法清晰易懂，其查詢計(jì)劃如下：

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 | derived2> | NULL | ref | auto_key0> | auto_key0> | 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)

該查詢對(duì)t_source表進(jìn)行了一次全表掃描，同時(shí)用filesort對(duì)表按分區(qū)字段created_time、item_name進(jìn)行了排序。外層查詢從每個(gè)分區(qū)中保留一條數(shù)據(jù)。因?yàn)橹貜?fù)created_time和item_name的多條數(shù)據(jù)中可以保留任意一條，所以oevr中不需要使用order by子句。

從執(zhí)行計(jì)劃看，窗口函數(shù)去重語(yǔ)句似乎沒(méi)有消除嵌套查詢的變量去重好，但此方法實(shí)際執(zhí)行是最快的。

MySQL窗口函數(shù)說(shuō)明參見(jiàn)“https://dev.mysql.com/doc/refman/8.0/en/window-functions.html”。

三、多線程并行執(zhí)行

前面已經(jīng)將單條查重語(yǔ)句調(diào)整到最優(yōu)，但還是以單線程方式執(zhí)行。能否利用多處理器，讓去重操作多線程并行執(zhí)行，從而進(jìn)一步提高速度呢？比如我的實(shí)驗(yàn)環(huán)境是4處理器，如果使用4個(gè)線程同時(shí)執(zhí)行查重SQL，理論上應(yīng)該接近4倍的性能提升。

1. 數(shù)據(jù)分片

在生成測(cè)試數(shù)據(jù)時(shí)，created_time采用每條記錄加一秒的方式，也就是最大和在最小的時(shí)間差為50萬(wàn)秒，而且數(shù)據(jù)均勻分布，因此先把數(shù)據(jù)平均分成4份。

（1）查詢出4份數(shù)據(jù)的created_time邊界值

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）查看每份數(shù)據(jù)的記錄數(shù)，確認(rèn)數(shù)據(jù)平均分布

mysql> select case when created_time >= '2017-01-01' 
 ->  and created_time  '2017-01-02 10:43:20'
 ->  then '2017-01-01'
 ->  when created_time >= '2017-01-02 10:43:20'
 ->  and created_time  '2017-01-03 21:26:40'
 ->  then '2017-01-02 10:43:20'
 ->  when created_time >= '2017-01-03 21:26:40' 
 ->  and created_time  '2017-01-05 08:10:00'
 ->  then '2017-01-03 21:26:40' 
 ->  else '2017-01-05 08:10:00'
 ->  end min_dt,
 -> case when created_time >= '2017-01-01' 
 ->  and created_time  '2017-01-02 10:43:20'
 ->  then '2017-01-02 10:43:20'
 ->  when created_time >= '2017-01-02 10:43:20'
 ->  and created_time  '2017-01-03 21:26:40'
 ->  then '2017-01-03 21:26:40'
 ->  when created_time >= '2017-01-03 21:26:40' 
 ->  and created_time  '2017-01-05 08:10:00'
 ->  then '2017-01-05 08:10:00'
 ->  else '2017-01-06 18:53:20'
 ->  end max_dt,
 -> count(*)
 -> from t_source
 -> group by case when created_time >= '2017-01-01' 
 ->  and created_time  '2017-01-02 10:43:20'
 ->  then '2017-01-01'
 ->  when created_time >= '2017-01-02 10:43:20'
 ->  and created_time  '2017-01-03 21:26:40'
 ->  then '2017-01-02 10:43:20'
 ->  when created_time >= '2017-01-03 21:26:40' 
 ->  and created_time  '2017-01-05 08:10:00'
 ->  then '2017-01-03 21:26:40' 
 ->  else '2017-01-05 08:10:00'
 ->  end,
 -> case when created_time >= '2017-01-01' 
 ->  and created_time  '2017-01-02 10:43:20'
 ->  then '2017-01-02 10:43:20'
 ->  when created_time >= '2017-01-02 10:43:20'
 ->  and created_time  '2017-01-03 21:26:40'
 ->  then '2017-01-03 21:26:40'
 ->  when created_time >= '2017-01-03 21:26:40' 
 ->  and created_time  '2017-01-05 08:10:00'
 ->  then '2017-01-05 08:10:00'
 ->  else '2017-01-06 18:53:20'
 ->  end;
+---------------------+---------------------+----------+
| min_dt  | max_dt  | count(*) |
+---------------------+---------------------+----------+
| 2017-01-01  | 2017-01-02 10:43:20 | 249999 |
| 2017-01-02 10:43:20 | 2017-01-03 21:26:40 | 250000 |
| 2017-01-03 21:26:40 | 2017-01-05 08:10:00 | 250000 |
| 2017-01-05 08:10:00 | 2017-01-06 18:53:20 | 250002 |
+---------------------+---------------------+----------+
4 rows in set (4.86 sec)

4份數(shù)據(jù)的并集應(yīng)該覆蓋整個(gè)源數(shù)據(jù)集，并且數(shù)據(jù)之間是不重復(fù)的。也就是說(shuō)4份數(shù)據(jù)的created_time要連續(xù)且互斥，連續(xù)保證處理全部數(shù)據(jù)，互斥確保了不需要二次查重。實(shí)際上這和時(shí)間范圍分區(qū)的概念類似，或許用分區(qū)表更好些，只是這里省略了重建表的步驟。

2. 建立查重的存儲(chǔ)過(guò)程

有了以上信息我們就可以寫(xiě)出4條語(yǔ)句處理全部數(shù)據(jù)。為了調(diào)用接口盡量簡(jiǎn)單，建立下面的存儲(chǔ)過(guò)程。

delimiter //
create procedure sp_unique(i smallint) 
begin 
 set @a:='1000-01-01 00:00:00'; 
 set @b:=' '; 
 if (i4) 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  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; 
 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 
//

查詢語(yǔ)句的執(zhí)行計(jì)劃如下：

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  date_add('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優(yōu)化器進(jìn)行索引范圍掃描，并且使用索引條件下推（ICP）優(yōu)化查詢。

3. 并行執(zhí)行

下面分別使用shell后臺(tái)進(jìn)程和MySQL Schedule Event實(shí)現(xiàn)并行。

（1）shell后臺(tái)進(jìn)程

•建立duplicate_removal.sh文件，內(nèi)容如下：

#!/bin/bash
mysql -vvv -u root -p123456 test -e "truncate t_target" >/dev/null 
date '+%H:%M:%S'
for y in {1..4}
do
 sql="call sp_unique($y)"
 mysql -vvv -u root -p123456 test -e "$sql" >par_sql1_$y.log 
done
wait
date '+%H:%M:%S'

•執(zhí)行腳本文件

./duplicate_removal.sh

執(zhí)行輸出如下：

[mysql@hdp2~]$./duplicate_removal.sh
14:27:30
14:27:35

這種方法用時(shí)5秒，并行執(zhí)行的4個(gè)過(guò)程調(diào)用分別用時(shí)為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，每個(gè)過(guò)程的執(zhí)行時(shí)間均4.85，因?yàn)槭遣⑿袌?zhí)行，總的過(guò)程執(zhí)行時(shí)間為最慢的4.91秒，比單線程速度提高了2.5倍。

（2）MySQL Schedule Event

•建立事件歷史日志表

-- 用于查看事件執(zhí)行時(shí)間等信息
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
);

•為每個(gè)并發(fā)線程創(chuàng)建一個(gè)事件

delimiter //
create event ev1 on schedule at current_timestamp + interval 1 hour on completion preserve disable do 
begin
 declare 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) 
 #作業(yè)名 
 values(database(),'ev1', v_starttime,v_randno); 
 begin 
 #異常處理段 
 declare continue handler for sqlexception 
 begin 
  set v_error = 1; 
  get diagnostics condition 1 r_code = returned_sqlstate , r_msg = message_text; 
 end; 
 #此處為實(shí)際調(diào)用的用戶程序過(guò)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 
begin
 declare 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) 
 #作業(yè)名 
 values(database(),'ev2', v_starttime,v_randno); 
 begin 
 #異常處理段 
 declare continue handler for sqlexception 
 begin 
  set v_error = 1; 
  get diagnostics condition 1 r_code = returned_sqlstate , r_msg = message_text; 
 end; 
 #此處為實(shí)際調(diào)用的用戶程序過(guò)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 
begin
 declare 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) 
 #作業(yè)名 
 values(database(),'ev3', v_starttime,v_randno); 
 begin 
 #異常處理段 
 declare continue handler for sqlexception 
 begin 
  set v_error = 1; 
  get diagnostics condition 1 r_code = returned_sqlstate , r_msg = message_text; 
 end; 
 #此處為實(shí)際調(diào)用的用戶程序過(guò)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 
begin
 declare 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) 
 #作業(yè)名 
 values(database(),'ev4', v_starttime,v_randno); 
 begin 
 #異常處理段 
 declare continue handler for sqlexception 
 begin 
  set v_error = 1; 
  get diagnostics condition 1 r_code = returned_sqlstate , r_msg = message_text; 
 end; 
 #此處為實(shí)際調(diào)用的用戶程序過(guò)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
//

為了記錄每個(gè)事件執(zhí)行的時(shí)間，在事件定義中增加了操作日志表的邏輯，因?yàn)槊總€(gè)事件中只多執(zhí)行了一條insert，一條update，4個(gè)事件總共多執(zhí)行8條很簡(jiǎn)單的語(yǔ)句，對(duì)測(cè)試的影響可以忽略不計(jì)。執(zhí)行時(shí)間精確到毫秒。

•觸發(fā)事件執(zhí)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;"

該命令行順序觸發(fā)了4個(gè)事件，但不會(huì)等前一個(gè)執(zhí)行完才執(zhí)行下一個(gè)，而是立即向下執(zhí)行。這可從命令的輸出可以清除看到：

[mysql@hdp2~]$mysql -vvv -u root -p123456 test -e "truncate t_target;alter event ev1 on schedule at current_timestamp enable;alter event ev2 on schedule at current_timestamp enable;alter event ev3 on schedule at current_timestamp enable;alter event ev4 on schedule at current_timestamp enable;"
mysql: [Warning] Using a password on the command line interface can be insecure.
--------------
truncate t_target
--------------
Query OK, 0 rows affected (0.06 sec)
--------------
alter event ev1 on schedule at current_timestamp enable
--------------
Query OK, 0 rows affected (0.02 sec)
--------------
alter event ev2 on schedule at current_timestamp enable
--------------
Query OK, 0 rows affected (0.00 sec)
--------------
alter event ev3 on schedule at current_timestamp enable
--------------
Query OK, 0 rows affected (0.02 sec)
--------------
alter event ev4 on schedule at current_timestamp enable
--------------
Query OK, 0 rows affected (0.00 sec)
Bye
[mysql@hdp2~]$

•查看事件執(zhí)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，每個(gè)過(guò)程的執(zhí)行均為4.83秒，又因?yàn)槭遣⑿袌?zhí)行的，因此總的執(zhí)行之間為最慢的5.3秒，優(yōu)化效果和shell后臺(tái)進(jìn)程方式幾乎相同。

總結(jié)

以上所述是小編給大家介紹的將MySQL去重操作優(yōu)化到極致的操作方法,希望對(duì)大家有所幫助，如果大家有任何疑問(wèn)請(qǐng)給我留言，小編會(huì)及時(shí)回復(fù)大家的。在此也非常感謝大家對(duì)腳本之家網(wǎng)站的支持！
如果你覺(jué)得本文對(duì)你有幫助，歡迎轉(zhuǎn)載，煩請(qǐng)注明出處，謝謝！

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