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Questions tagged [isolation-level]

Isolation level defines what data an SQL transaction can view or access while other transactions work with the same data.

ACID standard defines 4 isolation levels - read uncommitted, read committed, repeatable read and serializable. The higher the isolation level is, the more is guaranteed that another transaction can't break yours, but the lower amount of concurrency the database can handle. MySQL and MSSQL support all isolation levels, while PostgreSQL and Oracle support only the 2 most common, read committed and serializable

Read uncommitted means that the transaction works with the latest data available. In this isolation level it's possible that a transaction reads data that is not yet committed and possibly will be rolled back and never exist.

Read committed is the most basic isolation level, which ensures that transactions only read data that is already saved. It is possible however for another transaction to modify the data after the first transaction has read it but before it has modified it.

Repeatable read ensures that any subsequent read of the data will return the same result as the first read, therefore eliminating the race condition described above.

Serializable ensures that transactions are run in such a way that the result is the same as they were run in sequence, not in parallel.

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Why "SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED" returns rows in different order?

I am getting the rows in different order when I use SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED in my stored procedure. Below is the query defined in the stored procedure. SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED; SELECT …
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Why deadlock occurs?

I use a small transaction which consists of two simple queries: select and update: SELECT * FROM XYZ WHERE ABC = DEF and UPDATE XYZ SET ABC = 123 WHERE ABC = DEF It is quite often situation when the transaction is started by two threads, and…
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Behavior of PostgreSQL isolation levels

I am reading through section 13.2 of the PostgreSQL Manual, but the textual descriptions found there are not enough clear, and lack examples. For instance the following two paragraphs are not clear to whom is learning PostgreSQL: INSERT with an ON…
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Run SET Statement in Dapper SqlBuilder or sp_exectuesql

I'm trying to set transaction isolation level in Dapper using SqlBuilder, but didn't seem to work. var builder = new SqlBuilder(); var sqlStatement = builder.AddTemplate(@" SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED SELECT * FROM…
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DB2 "With UR" Spring Data JPA

I am using a Spring Data Repository interface to retrieve data from DB2 (z/OS). I have a couple of methods that rely on the method signatues and one that relies on an explicit @Query. How do I make the SQL generated have the "WITH UR" clause? I…
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transaction isolation level good explanation

Does someone knows a good book where transaction isolation levels are explained with a lot of examples and advices in what case what isolation level to use?
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Why better isolation level means better performance in SQL Server

When measuring performance on my query I came up with a dependency between isolation level and elapsed time that was surprising to me READUNCOMMITTED - 409024 READCOMMITTED - 368021 REPEATABLEREAD - 358019 SERIALIZABLE - 348019 Left column is table…
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In hibernate how to programmatically set the isolation level of a transaction, or how to create two transactions with different isolation levels

I'm using hibernate 3.6 with MSSQL 2005, 2008, 2012. I would like to set the isolation level of a transaction created by the session, but I can't find any information about in. This is my code Session sess = factory.openSession(); Transaction…
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Transaction isolation level - choosing the right one

I'm a sql beginner and I need help concerning isolation levels of transactions. I need to know which isolation level is the best for the following situation and why: There are 3 tables in the database: Animals (that are registered by inserting a…
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"SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED" not taking? Or am I looking in the wrong way?

We have a problem with some database code that apparently executes with the wrong isolation level. In this particular part of the code, it is supposed to execute with "READ UNCOMMITTED" to minimize locks. Inconsistent data is OK at this…
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Isolation levels in oracle

I would like to know different isolation levels with respect to commit, and would also like to know about row-level and table-level lock.
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Do writes in SNAPSHOT isolation level block writes in another SNAPSHOT transaction in SQL Server 2008 R2

For SNAPSHOt isolation level in SQL Server 2008 R2, the following is mentioned in MSDN ADO.Net documentation: Transactions that modify data do not block transactions that read data, and transactions that read data do not block transactions that…
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Practical uses of Seralization Isolation level?

Under what scenarios we use SERIALIZABLE Isolation level? I have seen some answers on website that says, when you want transaction to be completely isolated, we usually go for this. What i would like to know from your own experience is that, when…
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Isolation level in C# or SQL - which one will be used?

I have set isolation level in C# code as readcommitted, and I am calling a stored procedure which is timing out due to some reason. The stored procedure does not have any set isolation level statement. In SQL Server, database level isolation level…
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Read Committed vs Read Uncommited if both transaction do not rollback

I am trying to understand read committed and read uncommitted isolation levels. I know that theoreticay read uncommitted allows dirty reads and read committed doesn't, but I still can't really understand. Considering Figure above, if none of the…
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