mvcc | 易学教程

PostgreSQL并发控制（MVCC, 事务，事务隔离级别）

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【推荐】2019 Java 开发者跳槽指南.pdf(吐血整理) >>> 基于PostgreSQL9.4 9.3中文文档： http://58.58.27.50:8079/doc/html/9.3.1_zh/mvcc.html 9.4中文文档： http://www.postgresql.org/docs/9.4/static/mvcc.html 本文描述PostgreSQL数据库系统在多个会话试图同时访问同一数据时的表现。并发控制的目标是为所有会话提供高效的访问，同时还要维护严格的数据完整性。每个数据库应用开发人员都应该熟悉本话题。 PostgreSQL的MVCC与锁 PostgreSQL为开发者提供了丰富的对数据并发访问进行管理的工具。在内部， PostgreSQL利用多版本并发控制(MVCC)来维护数据的一致性。这就意味着当检索数据时，每个事务看到的都只是一小段时间之前的数据快照(一个数据库版本 )，而不是数据的当前状态。这样，如果对每个数据库会话进行事务隔离，就可以避免一个事务看到其它并发事务的更新而导致不一致的数据。MVCC通过避开传统数据库系统锁定的方法，最大限度地减少锁竞争以允许合理的多用户环境中的性能。使用MVCC与使用锁定模式相比较的优缺点：在MVCC里，对检索(读)数据的锁请求与写数据的锁请求不冲突，所以读不会阻塞写，而写也从不阻塞读。甚至当通过创新的

PostgreSQL之MVCC

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【推荐】2019 Java 开发者跳槽指南.pdf(吐血整理) >>> MVCC的介绍 MVCC技术，是multiversion concurrency control的缩写，翻译成中文是多版本并发控制的意思。目前多数DB都采用了这一技术，比如Oracle,Innodb,Berkeley DB等，采用这种方法可有效降低锁的开销。 MVCC的实现 MVCC是干嘛的呢？举例来说，当数据库做一个更新操作时，并不是将老的数据删除，再将新的数据覆盖上去，相反，它会把老的数据做一个标记隔离出去(old version)，然后再新增新的数据作为一个新的版本，这个时候新老数据都是存在数据库里的。但在不同的数据库里，其存储形式是不一样的。在PostgreSQL里，新老版本数据是混在一起的，Oracle和Innodb里是分开存储的，像Oracle有UNDO作为区分。或者说得更简单一点，读不会阻塞写操作，反过来也是一样。两种形式各有优缺点。 MVCC下的读事务通常以时间点或增长的事务ID来标记数据库里从个点开始读以及这个点所对应的数据。在PostgreSQL中，每一个事务都会获得一个事务ID,叫 XID, 这个可以是一个insert,update,delete或者是用begin...end包裹起来的一组SQL。当这个事务开始的时候，Postgres会增加XID值并赋给当前的事务

Transaction Isolation Across Multiple Tables using PostgreSQL MVCC

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问题 Question Summary This is a question about serializability of queries within a SQL transaction. Specifically, I am using PostgreSQL. It may be assumed that I am using the most current version of PostgreSQL. From what I have read, I believe the technology used to support what I am trying to do is known as "MultiVersion Concurrency Control", or "MVCC". To sum it up: If I have one primary table, and more-than-1 foreign-key-linked table connected to that primary table, how do I guarantee that, for

Changin DB transaction control in flyway with hsql

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问题 In HSQL to change TRANSACTION CONTROL there can't be any active transactions. Flyway, in turn, after committing migration X and before executing SQL from migration X, sets autocommitt=false and executes some of its own statements. So if the migration contains SET DATABASE TRANSACTION CONTROL statement it will wait for those uncommitted statements forever causing application to hang. (Side note: The statements executed by flyway before migration varies from version to version e.g. in 1.7 that

does Firebird defrag? If so, like a clustered index?

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问题 I've seen a few (literally, only a few) links and nothing in the documentation that talks about clustering with Firebird, that it can be done. Then, I shot for the moon on this question CLUSTER command for Firebird?, but answerer told me that Firebird doesn't even have clustered indexes at all, so now I'm really confused. Does Firebird physically order data at all? If so, can it be ordered by any key, not just primary, and can the clustering/defragging be turned on and off so that it only

Time when PostgreSQL ISOLATION LEVEL takes effect seems to be after first SELECT

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问题 I'm running PostgreSQL 9.5.3. I am trying to understand why I see a difference in behavior between the two routines below. I find this behavior counter-intuitive, but there may be a very good reason for it; I just want to know what it is if so. Setting ISOLATION LEVEL REPEATABLE READ does not seem to take effect until after the first SELECT statement. The only difference between the two routines is that in "Routine 2" I put in a superfluous SELECT 1 ; statement, whereas in "Routine 1" I did

High level Postgres run down of INSERT/UPDATE speed?

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问题 I know that when I UPDATE a row in Pg, that row gets rewritten and the old row gets deactivated when the new row gets activated. I know this is due to how the MVCC layer is implemented. What the advantages then of UPDATE over DELETE ... INSERT ? Is there anything else to be said about the relationship of UPDATE and DELETE in Postgresql? I can't find any docs that speak of this or mailing list postings.. Obviously, they run different user triggers if any but what happens under the hood to give

How Indices Cope with MVCC?

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问题 Greetings Overflowers, To my understanding (and I hope I'm not right) changes to indices cannot be MVCCed. I'm wondering if this is also true with big records as copies can be costly. Since records are accessed via indices (usually), how MVCC can be effective ? Do, for e.g., indices keep track of different versions of MVCCed records ? Any recent good reading on this subject ? Really appreciated ! Regards 回答1: Index itself can have both the records which can be pruned before returning. So in

Do MVCC databases see inserted rows in mid-transaction?

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问题 Does MVCC database isolation mode allow in-progress transactions to see rows inserted (and committed) by other transactions? For example, given: Table names[id BIGINT NOT NULL, name VARCHAR(30), PRIMARY KEY(id), UNIQUE(name)] Transactions T1 and T2, T1: open transaction T2: open transaction T1: select * from names; insert into names(name) values("John"); // do something commit; T2: select * from names; insert into names values("John"); // do something commit; When does T2 first become aware

Is it possible to implement Multi-Version Concurrency Control (MVCC) on top of MongoDB?

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问题 MongoDB is to me a great database. However there are cases where I really need atomic multi-document transactions. For example to transfer things (like money or reputation) between accounts and this needs to either succeed completely or fail completely. I wonder if it would be possible to interact with MongoDB through a library implementing the MultiVersion Concurrency Control pattern. How bad would it be concerning performances? Would it be possible and profitable to use a hybrid approach,