Default isolation level in Microsoft SQL Server 2005

Question

What is the default isolation level in SQL Server 2005 and in which scenario would you want to change it ? And little explanation about the default isolation level would be appreciated.

Thanks in anticipation

Answer 1

Default = READ COMMITTED

You don't need to change it often. Some folk recommend "always use READ UNCOMMITTED" but this is dangerous.

As to why, I'd suggest Wikipedia but in summary there are 3 data read issues that can occur:

1. Dirty reads
2. Non-repeatable reads
3. Phantom reads

Each of these is removed as you increase isolation (to SERIALIZABLE) at the cost of reduced concurrency.

The default level prevents "Dirty reads" and frankly is good enough.