[HACKERS] 答复: [HACKERS] 答复: questions about concurrency control in Po stgresql
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:alvhe...@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 may be 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 lock to 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 Herrerahttp://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 -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
[HACKERS] 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 be 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 lock to 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: gsst...@gmail.com [mailto:gsst...@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 huangxcl...@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 -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
[HACKERS] 答复: questions about concurrency control in Postgresql
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 -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
[HACKERS] some questions in postgresql developping
Hello, I’m a student from Nankai University in China. Now I and my team do a project which aims to integrate XML to Postgresql. What I do is to complete the function of XML Update. Now I’m researching in concurrency control. I have read the code about the concurrency control for a long time and I’m confident that I know it much. But I am puzzled that why we need to lock tuple. I think locking transaction is sufficient. I don’t think the tuple lock is good at improving executing rate or anything. I am wishing for your reply. Best Regards, --Huang Xiaocheng --Database Information System Lab, Nankai University, China
[HACKERS] questions about concurrency control in Postgresql
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(...). 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. So, may you tell me why there is tuple lock in Postgresql ? Is the tuple lock necessary? Thanks, --Huang Xiaocheng --Database Information System Lab, Nankai University
