将一个更新划分为几个批次_MySQL-Mysql Tutorial-php.cn

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将一个更新划分为几个批次_MySQL

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Jun 01, 2016 pm 02:05 PM

rotransactionCandatarenew

问：我想以一次5,000行的方式批量更新一个大型表，但我不知道怎样分割数据。该表不包含增量数字或整数主键。怎样才能在保持良好性能的同时实现数据更新呢？

答：如果知道哪些行尚未被更新，您可以使用简单的谓词来排除已更新的行，ROWCOUNT设置可以帮助您批量分割数据。以下的代码清单说明了如何使用该设置：

SET ROWCOUNT 1000
WHILE (1=1) BEGIN
BEGIN TRANSACTION
UPDATE...set ...,MyLastUpdate='date',...WHERE
MyLastUpdate -- 更新1000未更新行
IF @@ROWCOUNT = 0
BEGIN
COMMIT TRANSACTION
BREAK
END
COMMIT TRANSACTION
END

在指定的行数返回后，ROWCOUNT将使SQL Server停止查询处理。这项技术很有用，因为它避免了大量更新所致的并发命中；更新中的行数越少，更新任务使其他用户不能访问该数据的可能性就越小。结合事务日志备份，这一方法还可以使您的事务日志的大小降至最低。

如果没有识别已更新行的机制，您可以使用游标遍历所有数据并提交每个x值。但是，游标占用服务器资源的时间通常要多于基于集合的语句。

—Microsoft SQL Server开发团队

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