Re: postgres transaction isolation when rollback

"Tomer Levinboim" <levinboim(dot)tomer(at)gmail(dot)com> writes:
> Suppose now that the order is like so
> transaction 1: UPDATE accounts SET balance = balance + 100.00 WHERE acctnum
> = 12345;
> transaction 2: UPDATE accounts SET balance = balance + 200.00 WHERE acctnum
> = 12345;
> transaction 1: transaction failure
> transaction 2: UPDATE accounts SET balance = balance - 200.00 WHERE acctnum
> = 7534;

> Would the balance of account 12345 increase by 200 or 300 ?

200. Transaction 2 will block at its first UPDATE waiting to see if
transaction 1 commits or not, and will then use the appropriate version
of the row as the starting point for its update.

(I'm assuming you're speaking of READ COMMITTED rules here --- if T2
is SERIALIZABLE then the answer is different.)

regards, tom lane