1. A machine-implemented method, the method comprising: receiving a query for data stored across a plurality of data systems; generating a plurality of first logical plans for executing the query; for each of the plurality of first logical plans: generating a first set of physical plans for executing the query on the plurality of data systems based on the respective first logical plan; determining an execution cost for each physical plan from the first set of physical plans; and selecting the physical plan with a lowest determined execution cost from the first set of physical plans; selecting a physical plan with a lowest execution cost from among the respective physical plans selected for each of the plurality of first logical plans; executing an operation from the query according to the physical plan selected from among the respective physical plans selected for each of the plurality of first logical plans; updating the query based on results from the executed operation; generating a plurality of second logical plans for executing the updated query and a respective second set of physical plans for each of the plurality of second logical plans; determining an execution cost for each physical plan from the respective second sets of physical plans for the plurality of second logical plans; selecting a physical plan with a lowest determined execution cost among the respective second sets of physical plans for the plurality of second logical plans; executing an operation from the updated query according to the physical plan selected from among the respective second sets of physical plans; updating the updated query based on results from the executed operation from the updated query; and repeating the steps for generating a plurality of second logical plans and respective second sets of physical plans, determining an execution cost for each physical plan from the second sets of physical plans, selecting a physical plan with a lowest determined execution cost among the second sets of physical plans, executing an operation from the updated query, and updating the updated query until the updated query is complete.