1. A method for adaptive vector size selection for vectorized query execution, the method comprising: during a query planning time, at a query planner module, generating a query plan tree; splitting the query plan tree into a plurality of segments; calculating an initial vector size for each of the plurality of segments; assigning a vector size to each of the plurality of segments, wherein assigning the vector size to each of the plurality of segments comprises assigning the initial vector size to each of the plurality of segments; during a query execution time for the query plan tree, at a query execution engine, monitoring execution performance; and adjusting the vector size vector size according to the monitored execution performance.