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2014-05-16
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delete from mytable 必死无疑，你需要分批删除，尽量缩小每个批次删除的记录数，delete 是可以并行执行的，你可以同时运行多个删除操作
mysql> show processlist;+--------+-----------------+---------------------+-----------+---------+-------+-----------------------------+--------------------------------------------------------+| Id | User | Host | db | Command | Time | State | Info |+--------+-----------------+---------------------+-----------+---------+-------+-----------------------------+--------------------------------------------------------+| 1 | event_scheduler | localhost | NULL | Daemon | 52 | Waiting for next activation | NULL || 115986 | dba | localhost | example | Query | 0 | NULL | show processlist || 117446 | dba | localhost | example | Query | 20 | updating | delete from mytable where OPEN_TIME like '2011.11.28%' || 117525 | dba | localhost | example | Query | 2 | updating | delete from mytable where OPEN_TIME like '2011.12.02%' || 117526 | dba | localhost | example | Query | 49 | updating | delete from mytable where OPEN_TIME like '2011.12.12%' || 117527 | dba | localhost | example | Query | 6 | updating | delete from mytable where OPEN_TIME like '2011.12.21%' || 117528 | dba | localhost | example | Query | 64 | updating | delete from mytable where OPEN_TIME like '2011.12.30%' || 117546 | dba | localhost | example | Query | 33 | updating | delete from mytable where OPEN_TIME like '2011.11.10%' |+--------+-----------------+---------------------+-----------+---------+-------+-----------------------------+--------------------------------------------------------+23 rows in set (0.00 sec)
在电商领域常常遇到一个问题“调价”，经常需要调整一批商品的价格, 程序猿一条预警搞定有没有？
update goods set price=price+10 where category_id = xxx
在开发，测试环境是可以通过测试的，一旦部署到生产环境，必死无疑
大表创建索引需要很久的时间，通常要经历 manage keys 与 copy to tmp table 的过程
mysql> show processlist;+--------+-----------------+---------------------+----------+---------+-------+-----------------------------+------------------------------------------------------------------+| Id | User | Host | db | Command | Time | State | Info |+--------+-----------------+---------------------+----------+---------+-------+-----------------------------+------------------------------------------------------------------+| 1 | event_scheduler | localhost | NULL | Daemon | 47 | Waiting for next activation | NULL || 115986 | dba | localhost | example | Query | 0 | NULL | show processlist || 118814 | dba | 192.168.6.20:50459 | example | Query | 8 | copy to tmp table | ALTER TABLE `mytable` ADD INDEX `modifiy_time` (`MODIFY_TIME`) |+--------+-----------------+---------------------+----------+---------+-------+-----------------------------+------------------------------------------------------------------+17 rows in set (0.00 sec)
删除索引，也需要经理 copy to tmp table 过程，漫长的等待
mysql> show processlist;+--------+-----------------+---------------------+--------------+---------+-------+-----------------------------+-------------------------------------------------+| Id | User | Host | db | Command | Time | State | Info |+--------+-----------------+---------------------+--------------+---------+-------+-----------------------------+-------------------------------------------------+| 1 | event_scheduler | localhost | NULL | Daemon | 11 | Waiting for next activation | NULL || 115986 | dba | localhost | example | Query | 0 | NULL | show processlist || 118814 | dba | 192.168.6.20:50459 | example | Query | 4 | copy to tmp table | ALTER TABLE `mytable` DROP INDEX `modifiy_time` |+--------+-----------------+---------------------+--------------+---------+-------+-----------------------------+-------------------------------------------------+17 rows in set (0.00 sec)
所以数据设计要深思熟虑，做到提前未雨绸缪，不要亡羊补牢
OPTIMIZE 的操作是将当前表复制到临时表操作后再删除当前表，最后将临时表改名
mysql> show processlist;+--------+-----------------+---------------------+---------------------------+---------+-------+-----------------------------+--------------------------+| Id | User | Host | db | Command | Time | State | Info |+--------+-----------------+---------------------+---------------------------+---------+-------+-----------------------------+--------------------------+| 1 | event_scheduler | localhost | NULL | Daemon | 14 | Waiting for next activation | NULL || 115835 | dba | 192.168.6.20:49664 | example | Query | 9 | copy to tmp table | OPTIMIZE TABLE `mytable` || 115986 | dba | localhost | example | Query | 0 | NULL | show processlist |+--------+-----------------+---------------------+---------------------------+---------+-------+-----------------------------+--------------------------+17 rows in set (0.00 sec)
使用各种手段保证select操作不被受阻，只要select一直可以查询网站前端就能提供80%的功能，一旦select受阻一切都是浮云。
保证 select 操作优先于其他操作
UPDATE [LOW_PRIORITY] [IGNORE] tbl_name SET col_name1=expr1 [, col_name2=expr2 ...] [WHERE where_definition] [ORDER BY ...] [LIMIT row_count]
update的时候增加 LOW_PRIORITY 参数，可以降低更新语句的优先级。
my.cnf
[mysqld] low_priority_updates=1
或者启动是添加--low-priority-updates参数
全局开启
SET @@global.low_priority_updates = 1;
适用于本次会话连接
SET @@session.low_priority_updates = 1;
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