![]() But I think that this subtle difference make both samples not comparable. Or, otherwise, if you had written “WHERE i=1 ” instead of “WHERE Color=’White’ ” in your SESSION 2 for the first scenario, it would have updated the marble to black as well. If you would had made the join by color (u.color = m.color) the update wouldn’t had found the white marble, as it happened in the first scenario. This won’t happen by the fact that you are using a temp table. This will make the second update to work always, reagardless of what the first session does, and, thus, to change the marble color to black. It is 100% clear after your explanation that session 2 has to wait for session 1, and then it won’t find a white marble, because it will be already red, so nothing else will happen, and the marble will stay red.īut in your second scenario, you insert into the temp table, and then JOIN with the UpdateTest table by column “i” and not by “Color”. In your first scenario, you try to update in both session 1 and 2 the white marble. But I see your samples a little bit messed up. I don’t think that Read Committed Snapshot Isolation is any more complicated than plain old Read Committed, it’s just that people don’t think too much about Read Committed, since it’s the default! When you talk about any isolation level in detail in SQL Server, things get complicated. This May Sound Complicated, But It’s Not That Bad If you want to test that out, you can modify the code samples above to reproduce an “update conflict” with both of the sessions using SNAPSHOT isolation. There’s another isolation level, SNAPSHOT, which behaves a bit differently. By using the read-committed snapshot isolation level introduced in Microsoft SQL Server 2005, you can reduce blocking and deadlocking issues caused by lock contention. In this post we’ve looked at Read Committed Snapshot Isolation. Which might cause a lot of blocking, just like anytime you use locking hints. If you need to use a temporary table for performance reasons, but want your read to be blocked if any writer has a lock, you can achieve that by using locking hints. You just have to know how they behave so you can pick the right code for your situation. I’m not saying that it’s always better to do a single UPDATE or that it’s always better to use a temp table. Session #2 changes the color of the marble. ![]() When Session #1 completes, Session #2 is unblocked and updates using the data it’s cached in its temp table.But Session #2 becomes blocked at the point it runs the UPDATE.Session #2 is doing a SELECT statement, so it uses the version store and isn’t blocked.Session #1 locks the row, that’s the same.Serializable vs.In this case, Session #2 wasn’t completely blocked! Things happened differently:.Deadlocked!: "read committed snapshot" Explained (Nick Berardi)."The Potential Dangers of the Read Committed Snapshot Isolation Level" (JimMcLeod, disputed in comments by Alex Kuznetsov).The upside to this transfer is better performance. In general, therefore, snapshot isolation puts some of the problem of maintaining non-trivial constraints onto the user, who may not appreciate either the potential pitfalls or the possible solutions. The next section is "Making Snapshot Isolation Serializable" to get around this. Read "Snapshot isolation" (Wikipedia) for more on write-skew anomalies. Most important, snapshot isolations are not safe in many cases by default. "Performance Impact: The Potential Cost of Read_Committed_Snapshot" (Linchi Shea).It will also increase load on your tempdb and CPU. If you have locking problems then you have a problem with your code: it isn't the database engine.
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