read-uncommitted | 易学教程

In MySQL Why does setting a variable from a select acquire a lock when using read uncommitted?

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问题 We have a table in MySQL using InnoDB, and we are using a transaction isolation level of read uncommitted. Why does setting @x as shown acquire a lock? mysql> set @x = (select userID from users limit 1); Query OK, 0 rows affected (0.02 sec) mysql> Trying to update this table from another prompt results in a timeout error: mysql> update users set userID = 1; ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction 回答1: For what it's worth, this locking is not limited to READ

In MySQL Why does setting a variable from a select acquire a lock when using read uncommitted?

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How to set isolation level in @Transactional “READ_UNCOMMITTED”. I am using EclipseLink 2.5.1-RC1

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问题 I have a requirement to start new Transaction within an ongoing Transaction so that an exception in 2nd transaction will rollback only new transaction not the old one. This I am doing by setting propagation attribute in 2nd transaction like this: @Transactional(propagation = Propagation.REQUIRES_NEW) This created a new Transaction, but the new Transaction needs to read some uncommitted data of the first transaction (dirty read), and also update that data. This I am trying to do by setting

Read uncomitted data from HasMany-relationship with NHibernate

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问题 I have an NHibernate map which defines a HasMany-relationship on a type, i.e. a class has a list of another class. I would like NHibernate to be able to read uncommitted data including the list resulting from the HasMany-relationship. I have isolationlevel ReadUncomitted and I am able to write data and read it back before committing. However, the list is always empty, unless I commit first. Is there a way to make NHibernate populate objects with data from HasMany-relationships? EDIT It turns

Can ReadUncommitted know all LOWER auto-increment IDs that will exist?

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问题 Suppose we have a table with an auto-increment primary key. I want to load all IDs greater than the last ID I have seen. SELECT id FROM mytable WHERE id > 10; With the naive approach, I risk skipping IDs: Transaction 1 claims ID 11. Transaction 2 claims ID 12. Transaction 2 commits. I read all IDs >10. I see 12, and next time I will read all IDs >12. I have skipped 11. Transaction 1 commits. For all intents and purposes, ID 11 now exists. As a solution, I propose to do a double check to

NHibernate: populate list with uncommitted data related only by FK in database

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问题 This is an elaboration and clarification of this question. Suppose I have two tables, Foo and Bar . Bar has a FK to Foo . In the application, the tables are represented by classes and Foo has a list of Bar s. Bar has a property for the id of the Foo it has a FK to in the database. In the context of a Session and Transaction with IsolationLevel.ReadUncommitted , I add an instance of Foo to the database, assign the generated id to the Foo_id property of an instance of Bar and also add it to the

Can't see rows inserted by a running transaction when isolation level is READ_UNCOMMITTED

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问题 I have applications that insert rows into table A concurrently. Each application inserts rows in batch mode (using a JDBC prepared statement) using a single transaction per batch (to avoid rebuilding index after each INSERT ). The rows present in each batch are completely independent, the transaction is used only for optimization. Each inserted row has its primary key set automatically ( AUTO_INCREMENT ). I have another application that processes the rows from table A based on their IDs. The

High volume site using ADO.NET TransactionScope vs ExecuteCommand on NOLOCK, READ UNCOMMITTED directly?

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问题 Just read this interesting article by Omar on his blog Linq to SQL solve Transaction deadlock and Query timeout problem using uncommitted reads and at the end Javed Hasan started arguing with him about his solution to the nolock situation on a high volume site. Here, the problem trying to solve is, from the sql sense we need to use Select statements with NOLOCK or use SET TRANSACTION LEVEL READ UNCOMMITTED, otherwise at high volume rows in DB will be locked and cause errors. The technology

How to use WITH(NOLOCK) in LINQ to SQL?

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问题 we can use SQL just like this: SELECT * FROM student WITH(NOLOCK); How can I achieve this with LINQ to SQL without the use of a TransactionScope ? 回答1: LINQ to SQL does not have any mechanism of doing this, but you can create a transaction with a specific isolation level. Look at the code below: using (var con = new SqlConnection("constr")) { con.Open(); using (var transaction = con.BeginTransaction( IsolationLevel.ReadUncommitted)) { using (var context = new SchoolDataContext(con)) { // HACK

High volume site using ADO.NET TransactionScope vs ExecuteCommand on NOLOCK, READ UNCOMMITTED directly?

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Just read this interesting article by Omar on his blog Linq to SQL solve Transaction deadlock and Query timeout problem using uncommitted reads and at the end Javed Hasan started arguing with him about his solution to the nolock situation on a high volume site. Here, the problem trying to solve is, from the sql sense we need to use Select statements with NOLOCK or use SET TRANSACTION LEVEL READ UNCOMMITTED, otherwise at high volume rows in DB will be locked and cause errors. The technology Omar used is Linq2Sql, so the question is how do we get this achieved in your C# data access code so the