SELECT ... FOR UPDATE *的目的是什么? [英] What's the purpose of SELECT ... *FOR UPDATE*?

问题描述

我对为什么要指定FOR UPDATE感到困惑-为什么数据库为什么要处理SELECT中的数据呢?

I'm confused as to why you would specify FOR UPDATE -- why does the database care what you're going to do with the data from the SELECT?

对不起，我问的问题很差.我知道文档说这会将事情变成锁定读取"-我想知道的是在什么情况下，在指定FOR UPDATE与不指定FOR UPDATE之间，可观察到的行为会有所不同-也就是说，专门需要锁定吗?

Sorry, I asked the question poorly. I know the docs say that it turns things into a "locking read" -- what I'd like to know is "what cases exist where the observable behavior will differ between specifying FOR UPDATE and not specifying it -- that is, what specifically does that lock entail?

推荐答案

http ://dev.mysql.com/doc/refman/5.0/en/innodb-locking-reads.html

这与在事务中锁定表有关.假设您具有以下条件:

It has to do with locking the table in transactions. Let's say you have the following:

START TRANSACTION;
SELECT .. FOR UPDATE;
UPDATE .... ;
COMMIT;

在SELECT语句运行之后，如果您还有其他用户的另一个SELECT，则直到您的第一个事务到达COMMIT行时，它才会运行.

after the SELECT statement runs, if you have another SELECT from a different user, it won't run until your first transaction hits the COMMIT line.

还请注意，在交易之外的FOR UPDATE是没有意义的.

Also note that FOR UPDATE outside of a transaction is meaningless.

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