I'd say MOST non trivial application require SERIALIZABLE. Every time apps does `BEGIN; SELECT WHERE; INSERT/UPDATE; COMMIT` it needs `SERIALIZABLE`, becuase it

I'd say MOST non trivial application require SERIALIZABLE. Every time apps does `BEGIN; SELECT WHERE; INSERT/UPDATE; COMMIT` it needs `SERIALIZABLE`, becuase it is only level catching cases, where concurrent transaction adds rows so that SELECT WHERE changes it's result set and therefore subsequent INSERT/UPDDATE should be done with different values.