Implementing Projection
•Two parts:
•    (1) remove unwanted attributes,
•    (2) remove duplicates from the result.
•Refinements to duplicate removal:
–If an index on a relation contains all wanted attributes, then we can do an index-only scan.
–If the index contains a subset of the wanted attributes, you can remove duplicates locally.
SELECT   DISTINCT
               R.name,        
               R.phone
FROM Person R