performance of transaction isolation serializable / optimistic locking

Hi,

I realize that there is a performance hit when you use serializable transaction, but I'm looking for a better feel of how much the hit is. I also realize this is a vague question since it'll depend on the ratio of writes to reads, the likelyhood of conflict, etc., but what are people's experiences with serializable transaction. Switching from default read committed to serializable what are "typical" performance hits you have experience. It seems that since Oracle doesn't put read locks for either serializable or read committed the hit in oracle for serializable may be significantly less than compared to other databases that uses read locks. Is this true. My issue is how to get optmistic locking. Seems that either to serialize the transaction or do verified updates. If the hit isn't that big I'd rather not have to make my sql verified the updates (i.e., update table set attribute = 'value' where version = x).

Much thanks.

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Tinou Bao
www.tinou.com