PERF: fewer objects, pass through on values

lib/arel/algebra/relations/relation.rb

@@ -65,7 +65,14 @@ def primary_key
     end
 
     def select_clauses
-      attributes.collect { |a| a.to_sql(Sql::SelectClause.new(self)) }
+      attributes.map { |a|
+        case a
+        when Value
+          a.value
+        else
+          a.to_sql(Sql::SelectClause.new(self))
+        end
+      }
     end
 
     def from_clauses