MySQL根据主键切割大事务(变相ROWID切片)

Posted 2022-04-09 robinson1988

不管是Oracle，mysql，还是PostGre SQL，跑大事务都会严重影响数据库性能

在Oracle里面可以利用rowid切片的方式处理大事务，如果不清楚什么是rowid切片，可以买一本《SQL优化核心思想》看看

MySQL没有Oracle的区，块，段这些概念，所以也就不能使用rowid切片的方式处理大事务

MySQL中表一般都是存在innodb引擎中的，使用innodb引擎每个表都必须要设置主键(当然了你也可以不设置主键，你屌)

MySQL中主键一般都是自增(auto_increment)的，所以可以模仿Oracle的做法来变相实现ROWID切片

MySQL跑大事务危害非常大，DBA最担心的就是主从同步问题，MySQL里面跑个大事务很容易引起主从同步延迟

现在有个表t，主键为id，最小值和最大值如下：

mysql> select min(id) from t;
+---------+
| min(id) |
+---------+
|       2 |
+---------+
1 row in set (0.00 sec)

mysql> select max(id) from t;
+---------+
| max(id) |
+---------+
|  365926 |
+---------+
1 row in set (0.00 sec)

要执行update t set owner='沙雕';

可以根据主键对表切割，比如切为50份：

mysql> SELECT concat('update t set owner= ''沙雕'' where id>',
    ->               avg_row * (n - 1),
    ->               ' and id<=',
    ->               avg_row * n,
    ->               ';') split_sql
    ->   FROM (SELECT n, min_id, max_id, ceil(max_id / 50) avg_row
    ->           FROM (WITH RECURSIVE x(n) AS (SELECT 1 UNION ALL
    ->                                                 SELECT n + 1
    ->                                                   FROM x
    ->                                                  WHERE n < 50
    ->                                         )
    ->                  SELECT *
    ->                    FROM x) a, (SELECT min(id) min_id FROM t) b, (SELECT max(id) max_id
    ->                                                                           FROM t) c
    ->         ) a;
+--------------------------------------------------------------+
| split_sql                                                    |
+--------------------------------------------------------------+
| update t set owner= '沙雕' where id>0 and id<=7319;          |
| update t set owner= '沙雕' where id>7319 and id<=14638;      |
| update t set owner= '沙雕' where id>14638 and id<=21957;     |
| update t set owner= '沙雕' where id>21957 and id<=29276;     |
| update t set owner= '沙雕' where id>29276 and id<=36595;     |
| update t set owner= '沙雕' where id>36595 and id<=43914;     |
| update t set owner= '沙雕' where id>43914 and id<=51233;     |
| update t set owner= '沙雕' where id>51233 and id<=58552;     |
| update t set owner= '沙雕' where id>58552 and id<=65871;     |
| update t set owner= '沙雕' where id>65871 and id<=73190;     |
| update t set owner= '沙雕' where id>73190 and id<=80509;     |
| update t set owner= '沙雕' where id>80509 and id<=87828;     |
| update t set owner= '沙雕' where id>87828 and id<=95147;     |
| update t set owner= '沙雕' where id>95147 and id<=102466;    |
| update t set owner= '沙雕' where id>102466 and id<=109785;   |
| update t set owner= '沙雕' where id>109785 and id<=117104;   |
| update t set owner= '沙雕' where id>117104 and id<=124423;   |
| update t set owner= '沙雕' where id>124423 and id<=131742;   |
| update t set owner= '沙雕' where id>131742 and id<=139061;   |
| update t set owner= '沙雕' where id>139061 and id<=146380;   |
| update t set owner= '沙雕' where id>146380 and id<=153699;   |
| update t set owner= '沙雕' where id>153699 and id<=161018;   |
| update t set owner= '沙雕' where id>161018 and id<=168337;   |
| update t set owner= '沙雕' where id>168337 and id<=175656;   |
| update t set owner= '沙雕' where id>175656 and id<=182975;   |
| update t set owner= '沙雕' where id>182975 and id<=190294;   |
| update t set owner= '沙雕' where id>190294 and id<=197613;   |
| update t set owner= '沙雕' where id>197613 and id<=204932;   |
| update t set owner= '沙雕' where id>204932 and id<=212251;   |
| update t set owner= '沙雕' where id>212251 and id<=219570;   |
| update t set owner= '沙雕' where id>219570 and id<=226889;   |
| update t set owner= '沙雕' where id>226889 and id<=234208;   |
| update t set owner= '沙雕' where id>234208 and id<=241527;   |
| update t set owner= '沙雕' where id>241527 and id<=248846;   |
| update t set owner= '沙雕' where id>248846 and id<=256165;   |
| update t set owner= '沙雕' where id>256165 and id<=263484;   |
| update t set owner= '沙雕' where id>263484 and id<=270803;   |
| update t set owner= '沙雕' where id>270803 and id<=278122;   |
| update t set owner= '沙雕' where id>278122 and id<=285441;   |
| update t set owner= '沙雕' where id>285441 and id<=292760;   |
| update t set owner= '沙雕' where id>292760 and id<=300079;   |
| update t set owner= '沙雕' where id>300079 and id<=307398;   |
| update t set owner= '沙雕' where id>307398 and id<=314717;   |
| update t set owner= '沙雕' where id>314717 and id<=322036;   |
| update t set owner= '沙雕' where id>322036 and id<=329355;   |
| update t set owner= '沙雕' where id>329355 and id<=336674;   |
| update t set owner= '沙雕' where id>336674 and id<=343993;   |
| update t set owner= '沙雕' where id>343993 and id<=351312;   |
| update t set owner= '沙雕' where id>351312 and id<=358631;   |
| update t set owner= '沙雕' where id>358631 and id<=365950;   |
+--------------------------------------------------------------+
50 rows in set (0.00 sec)