Can two transaction run at same time in Oracle?

I have a method

void SaveApplication开发者_JAVA技巧Data()
{
   begin transaction
   update
   insert
   commit transaction
}

if two users call this method at same time, can these two database transaction run at same time?

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Large numbers of transactions can be running simultaneously. However, if they're updating the same rows, they may well have locking problems, and one or more may be rolled back. (It's a long time since I dealt with conflicting transactions.)

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yes they can run at the same time

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Make sure of using the proper Isolation Level

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No it cannot run at the same time. Let say u have an account having k dollars and in first transaction u requested to withdraw k dollars and in 2nd transaction u requested same. U will get 2 k dollars because u did transaction at same time and your account balance shows k dollars for both transaction. so avoid this i think they use synchronization. without completing the first transaction u cannot go for 2nd and if u go u have to wait till completion of first.