Is it possible to somehow remove cache before a query execution?

This might a be a bit weird question but I am trying to measure the performance of Vertica against other database systems and other utilities in a specific application area. This measurement involves executing four different queries, each of them on basically the same data but with different scope. What I am trying to achieve is to execute each of them as if it was ad-hoc and no cache was involved in the process. In other words, it is clear that if I execute a query on a half of the rows of the table and consequently on the whole table, the second one will take comparatively less time. Is it possible to avoid this? Is it possible to somehow erase the cache before a query execution? Thank you. Adam