I ran a simple query over 21 Gb of data on an XL warehouse for 90 minutes before canceling it. After the first ten minutes, nothing changed significantly in the query profile, other than the ‘Bytes sent over the network’ metric, which had increased steadily the entire time and had surpassed 1000 Gb at the time that I cancelled the query. What could cause that much data to be transferred over the network, and what could I do to improve the performance of this query?