Consider this sequence on PostgreSQL at READ COMMITTED. What value does the final SELECT in session A return, and why?
-- Session A
BEGIN;
SELECT bal FROM acct WHERE id=1; -- returns 100
-- Session B (concurrent)
BEGIN;
UPDATE acct SET bal = 500 WHERE id=1;
COMMIT;
-- Session A continues
UPDATE acct SET bal = bal + 1 WHERE id=1;
SELECT bal FROM acct WHERE id=1; -- ?