FastExport for Really Short Queries in Teradata 13.10 - comment by carrie

Padhia,
 
The sessions access data in a round robin fashion across the AMPs, getting one block of rows at a time from each AMP, then moving on to the next AMP.   The client knows how to make sense of the returned data block and unpack the individual rows.  You could use only one session to accomplish this return of data, or several sessions.
 
The purpose of using more than one session is that then the client can do multi-buffering and can have more than one fetch outstanding at a time.  This usually speeds up the return of large volumes of data.  
 
If one session were active, it would fetch blocks in round robin manner across all AMPs by itself.  If two or more sessions are active, then those sessions can interleave their fetch activity so more total fetching can be done, usually in a shorter period of time.
 
Thanks, -Carrie