1. A method for retrieving data from a computerized database system having at least one processor, a memory, and at least one non-volatile storage system, using a query statement to identify data to be retrieved, comprising the steps performed by the database of: (a) during compile time: (i) determining an optimal sequential execution plan for the query statement, and (ii) determining an optimal parallel execution plan based on said optimal sequential execution plan, and based on available resource information; and (b) during run time: (i) determining, based on a value of at least one run time variable, whether to use said sequential execution plan without modification or a modification of said parallel execution plan as an optimal run time execution plan, and (ii) executing said query statement using said optimal run time execution plan.