Scan Operator

The Scan operator evaluates predicates and reads data from disk.  It could be slow because of an unoptimized projection.  Expensive predicates (regular expressions, date/time transformations, LIKE, etc ... ) will show up as high "execution time (us)".  

  --Sharon

Thanks Sharon. 

One more quick question : What exactly do the opeartors NetworkSend and NetworkRecv messages mean then ? What exactly is the difference between these operators and the Scan operator ?

Scan operator is reading the projection and evaluating predicates.  NetworkSend and NetworkRecv are moving data across the network.  For example if you are doing a join and the projections aren't segmented on the join keys.  Or a GROUP BY where the data isn't segmented by the groups.  

Thanks Sharon for the clarification.