I'm not sure if I get your point correctly. I guess you want to test Impala query performance. Usually, we can redirect the output to /dev/null or a local file but the network and disk IO throughput will affect the test results so you want the coordinator executes the query but not send the result back to the client. Is it correct?
As I know, there isn't such a feature in Impala coordinator to discard the rows when they are ready to be fetched by the client. You can try the query "CREATE TABLE new_table AS original_select_query". This query does the same as the original SELECT query does but save the result in a new HDFS table instead of sending back to the client. In this way, you can get a very close performance assessment of a SELECT query.