• relation is indexed on (A₁,A₂,...A_n)    (indexes described later)
• projected attributes are a prefix of (A₁,A₂,...A_n)

Basic idea:

• scan through index file (which is already sorted on attributes)
• duplicates are already adjacent in index, so easy to skip

• index has b_i pages   (where b_i ≪ b_R)
• Cost = b_i reads + b_Out writes