simp only [nrows, selectColumns1, Schema.length_eq_List_length]
η : Type u_η
dec_η : DecidableEq η
sch : Schema
t : Table sch
bs : List Bool
⊢ nrows (selectColumns1 t bs) = nrows t
η : Type u_η
dec_η : DecidableEq η
sch : Schema
t : Table sch
bs : List Bool
⊢ List.length (List.map (fun r => Row.sieve bs r) t.rows) = List.length t.rows