February 7, 2013 at 5:31 pm
HI,
Which is the most preferred isolation level in T-SQL? Does it change from case to case? Say does it differ from a OLAP,OLTP,web application or is it the same? what is the reason?
Thanks.
February 8, 2013 at 3:23 am
Default is READ COMMITTED, typically only change it if you are having problems, and I always stay away from READ UNCOMMITTED
February 13, 2013 at 1:30 pm
As was stated READ COMMITTED is the default iso level and the default iso level cannot be changed to something else. Once connected anyone can explicitly change the iso level for their session using the SET TRANSACTION ISOLATION LEVEL command.
READ_COMMITTED_SNAPSHOT isolation (RCSI) is an alternate behavioral mode of READ COMMITTED that is worth looking into if you're experiencing blocking in a database where both reads and writes occur simultaneously, e.g. in an OLTP system also used for some real-time reporting. I would strongly recommend RCSI over READ UNCOMMITTED in that scenario. There are very few use cases where READ UNCOMMITTED is safe.
It's not really a question of the type of system, e.g. OLTP, OLAP, etc., it's a question of how much concurrency do you want to support versus how much data-integrity protection you need.
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