答复: questions about concurrency control in Postgresql

I think I know why we need tuple lock.
Though we have tuple's infomask shows whether the tuple is being updated, before we set the tuple's infomask, there may
betwo transaction coming and updating the tuple. They both think the tuple is ok to be updated, and then it's wrong. 
In PostgreSQL, we can use buffer lock to solve the problem , but its granularity is not proper. So we must use tuple
lockto solve the problem. 
Thank you, Greg. You prompt me to think clearly about it.
Happy communicating with you, and thanks again.


--Huang Xiaocheng
--Database & Information System Lab, Nankai University

-----邮件原件-----
发件人: gsstark@gmail.com [mailto:gsstark@gmail.com] 代表 Greg Stark
发送时间: 2009年12月8日 20:16
收件人: 黄晓骋
抄送: pgsql-hackers@postgresql.org
主题: Re: questions about concurrency control in Postgresql

2009/12/8 黄晓骋 <huangxclife@gmail.com>:
> From the above, I think the tuple lock is unnecessary, because it uses
> transaction lock.
>
> Besides, tuple lock is unlocked after the tuple is updated but not after the
> transaction commits. I mean it's not 2PL.

It's a two step process. An update marks the tuple locked. Another
transaction which comes along and wants to lock the tuple waits on the
transaction marked on the tuple. When the first transaction commits or
aborts then the second transaction can proceed and lock the tuple
itself. The reason we need both locks is because the first transaction
cannot go around the whole database finding every tuple it ever locked
to unlock it, firstly that could be a very large list and secondly
there would be no way to do that atomically.

Tuple locks and all user-visible locks are indeed held until the end
of the transaction.

--
greg


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