Thread: questions about concurrency control in Postgresql
<div class="Section1"><p class="MsoNormal"><span lang="EN-US">Hello,</span><p class="MsoPlainText"><span lang="EN-US">I thinkin Postgresql, concurrency control acts like this:</span><p class="MsoPlainText"><span lang="EN-US">tuple's infomaskshows if it is being updated. If it is being updated now, the latter transaction should reread the tuple and getthe newer tuple. During the progress of getting the newer tuple, it must use transaction lock, I mean XactLockTableWait(...).</span><pclass="MsoPlainText"><span lang="EN-US">From the above, I think the tuple lock is unnecessary,because it uses transaction lock.</span><p class="MsoPlainText"><span lang="EN-US">Besides, tuple lock is unlockedafter the tuple is updated but not after the transaction commits. I mean it's not 2PL.</span><p class="MsoPlainText"><spanlang="EN-US">So, may you tell me why there is tuple lock in Postgresql ? Is the tuple lock necessary?</span><pclass="MsoPlainText"><span lang="EN-US"> </span><p class="MsoNormal"><span lang="EN-US">Thanks,</span><pclass="MsoNormal"><span lang="EN-US"> </span><p class="MsoNormal"><span lang="EN-US">--HuangXiaocheng</span><p class="MsoNormal"><span lang="EN-US">--Database & Information System Lab, NankaiUniversity</span><p class="MsoNormal"><span lang="EN-US"> </span></div><br /><br />__________ Information from ESETNOD32 Antivirus, version of virus signature database 4668 (20091207) __________<br /><br />The message was checked byESET NOD32 Antivirus.<br /><br /><a href="http://www.eset.com">http://www.eset.com</a><br />
2009/12/7 黄晓骋 <huangxclife@gmail.com>: > Hello, > > I think in Postgresql, concurrency control acts like this: > > tuple's infomask shows if it is being updated. If it is being updated now, > the latter transaction should reread the tuple and get the newer tuple. > During the progress of getting the newer tuple, it must use transaction > lock, I mean XactLockTableWait(...). That is a table lock...depending on the lock, other backends may be allowed to update tuples in the relation still. Fine-grained tuple locks are used to prevent unnecessary contention for a table-wide lock. See the documentation at the manual page: http://www.postgresql.org/docs/8.4/static/explicit-locking.html It gives a thorough treatment of table and row locking levels and conflicts, as well as what gets what locks. fdr
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
>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. I agree with it. >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. You mean that 2PL is hard to realize actually, I agree too. But it doesn't mean tuple lock is necessary. >Tuple locks and all user-visible locks are indeed held until the end >of the transaction. I don't agree with it, for I see unlocktuple(...) in heap_update(...). --Huang Xiaocheng --Database & Information System Lab, Nankai University __________ Information from ESET NOD32 Antivirus, version of virus signature database 4671 (20091208) __________ The message was checked by ESET NOD32 Antivirus. http://www.eset.com
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 __________ Information from ESET NOD32 Antivirus, version of virus signature database 4671 (20091208) __________ The message was checked by ESET NOD32 Antivirus. http://www.eset.com __________ Information from ESET NOD32 Antivirus, version of virus signature database 4674 (20091209) __________ The message was checked by ESET NOD32 Antivirus. http://www.eset.com
You are right. I never consider the SELECT FOR UPDATE/SHARE type queries, so I got the wrong conclusion. I have seen the content in the comment of heap_lock_tuple(). Thank you, Best Regards, --Huang Xiaocheng --Database & Information System Lab, Nankai University -----邮件原件----- 发件人: Alvaro Herrera [mailto:alvherre@commandprompt.com] 发送时间: 2009年12月10日 22:54 收件人: 黄晓骋 抄送: 'Greg Stark'; pgsql-hackers@postgresql.org 主题: Re: [HACKERS] 答复: questions about concurrency control in Postgresql 黄晓骋 escribió: > 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 maybe two 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. Actually it's the buffer lock that's used to protect most of infomask. Tuple locks are only used while XMAX and some infomask bits are set for SELECT FOR UPDATE/SHARE type queries. That can take a while because it may need I/O in pg_multixact, so the buffer lock is not appropriate to hold for so long. -- Alvaro Herrera http://www.CommandPrompt.com/ The PostgreSQL Company - Command Prompt, Inc. __________ Information from ESET NOD32 Antivirus, version of virus signature database 4677 (20091210) __________ The message was checked by ESET NOD32 Antivirus. http://www.eset.com __________ Information from ESET NOD32 Antivirus, version of virus signature database 4687 (20091214) __________ The message was checked by ESET NOD32 Antivirus. http://www.eset.com