check insert img in results
{
     if already seen img
          return  // no need to check
     
     data = dbm.lookup(Db , img)
     vimg = dbm.fetch(data)
     d = D(vimg , vq)
     include img in k-NN if d small enough
}

The dbm.lookup steps are expensive   (T_sel)

Once a cursor is established, access is via cached buffer.