Posted by Erland Sommarskog on 11/03/07 10:47

dhek (dhek@dhek.dk) writes:
> I have 1 SQL statement selecting data from various tables and updating
> other tables.
>
> The question then is how do I prevent other applications from modifying
> the tables that I'm working on (that is while my transaction is being
> executed)?
>
>
> I know that the isolation level should be either REPEATABLE READ or
> SERIALIZABLE. But I need confirmation on if one of these actually solve my
> issue - prevents other applications/threads from modifying/inserting data
> into the same tables that I'm working on.

It's difficult to give a single answer, since I don't know your exact
requirements, so I have to answer in genric terms.

If you want a consistent snapshot of how the data looks in this precise
moment, the isolation level you should use is snapshot isolation. Snapshot
isolation is available only in SQL 2005 and later. Furthermore the database
must be configured to permit snapshot isolation. When you have snapshot is
created when the transaction starts, or at latest when you start to read
data. If data is updated while your query runs, you will not see these
updates. This gives you a consistent view - but it may also give you
outdated data, depending on how you look at it.

On SQL 2000, snapshot isolation is not available, and the only foolproof
way to get consistent data, is to set the database in single-user mode.

In the default isolation level, READ COMMITTED, if you read the same
row twice, you may get different results in different accesses. For
instance, if you run:

SELECT O.OrderID, E.EmployeeID, E.LastName
FROM Orders O
JOIN Employees E ON O.EmployeeID = E.EmployeeID

You may see different last names for the same employee ID if the query
plan uses a loop join, and the last name is updated while the query is
running.

In the next level, REPEATABLE READ locks are held, and you are guaranteed
that reading the same row twice will yield the same result. However, if
the last name of employee 8 was Grønkjær when the query started, and
updated to Gravesen before you have read any orders with employee 8,
you would see Gravesen in the result set.

SERIALIZABLE adds protection against "phantom insert", so if you read the
same range twice, you will get the same result. That is, if you run
SELECT MAX(OrderID) FROM Orders twice in the same serializable transaction,
you will get the same result. But if a order is added after you started the
transaction, but before your query runs, the order will show up.


--
Erland Sommarskog, SQL Server MVP, esquel@sommarskog.se

Books Online for SQL Server 2005 at
http://www.microsoft.com/technet/prodtechnol/sql/2005/downloads/books.mspx
Books Online for SQL Server 2000 at
http://www.microsoft.com/sql/prodinfo/previousversions/books.mspx

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