(List.append sort_cols s, List.append rel_cols r, List.append obj_cols o))
([], [], []) ind_metadata
+(*
let pp_columns ?(sep = "\n") (sort_cols, rel_cols, obj_cols) =
String.concat sep
([ "Sort" ] @ List.map Dbi.sdebug (sort_cols :> Dbi.sql_t list list) @
[ "Rel" ] @ List.map Dbi.sdebug (rel_cols :> Dbi.sql_t list list) @
[ "Obj" ] @ List.map Dbi.sdebug (obj_cols :> Dbi.sql_t list list))
+*)