1 files changed, 22 insertions, 17 deletions

diff --git a/lib/drl/executor.ml b/lib/drl/executor.ml
index f1732eb..3ff4553 100644
--- a/lib/drl/executor.ml
+++ b/lib/drl/executor.ml
@@ -20,6 +20,19 @@ module Make (Storage : Management.Physical.S) = struct
   let wrap = Result.map_error (fun e -> AlgebraError e)
   let ast_value_to_abstract = Ast.value_to_abstract
 
+  let select_semijoin storage source filter =
+    let common =
+      List.filter_map
+        (fun (n, _) ->
+          if List.exists (fun (m, _) -> m = n) filter.Relation.schema
+          then Some n
+          else None)
+        source.Relation.schema
+    in
+    let source_attrs = List.map fst source.Relation.schema in
+    Result.bind (wrap (Alg.equijoin storage common source filter)) (fun joined ->
+        wrap (Alg.project storage source_attrs joined))
+
   let rec execute (storage : Storage.t) (db : Management.Database.t)
       (q : Ast.query) : (Relation.t, error) Result.t =
     let ( >>= ) = Result.bind in
@@ -35,40 +48,32 @@ module Make (Storage : Management.Physical.S) = struct
     | Ast.Select (filter_q, source_q) ->
         execute storage db filter_q >>= fun filter ->
         execute storage db source_q >>= fun source ->
-        (* semijoin = project(source attrs, equijoin on common attrs) *)
-        let common =
-          List.filter_map
-            (fun (n, _) ->
-              if List.exists (fun (m, _) -> m = n) filter.Relation.schema then
-                Some n
-              else None)
-            source.Relation.schema
-        in
-        let source_attrs = List.map fst source.Relation.schema in
-        wrap (Alg.equijoin storage common source filter) >>= fun joined ->
-        wrap (Alg.project storage source_attrs joined)
+        select_semijoin storage source filter
     | Ast.Join (attrs, q1, q2) ->
         execute storage db q1 >>= fun r1 ->
         execute storage db q2 >>= fun r2 ->
         wrap (Alg.equijoin storage attrs r1 r2)
     | Ast.Project (attrs, q) ->
-        execute storage db q >>= fun rel -> wrap (Alg.project storage attrs rel)
+        execute storage db q >>= fun rel ->
+        wrap (Alg.project storage attrs rel)
     | Ast.Rename (renames, q) ->
         execute storage db q >>= fun rel ->
         wrap (Alg.rename storage renames rel)
     | Ast.Cartesian (q1, q2) ->
         execute storage db q1 >>= fun r1 ->
         execute storage db q2 >>= fun r2 ->
-        (* Cartesian product = join on no attrs *)
         wrap (Alg.equijoin storage [] r1 r2)
     | Ast.Union (q1, q2) ->
         execute storage db q1 >>= fun r1 ->
-        execute storage db q2 >>= fun r2 -> wrap (Alg.union storage r1 r2)
+        execute storage db q2 >>= fun r2 ->
+        wrap (Alg.union storage r1 r2)
     | Ast.Diff (q1, q2) ->
         execute storage db q1 >>= fun r1 ->
-        execute storage db q2 >>= fun r2 -> wrap (Alg.diff storage r1 r2)
+        execute storage db q2 >>= fun r2 ->
+        wrap (Alg.diff storage r1 r2)
     | Ast.Take (n, q) ->
-        execute storage db q >>= fun rel -> wrap (Alg.take storage n rel)
+        execute storage db q >>= fun rel ->
+        wrap (Alg.take storage n rel)
 end
 
 module Memory = Make (Management.Physical.Memory)