continue to speed up datacolumn#split_me? issue #288

app/models/datacolumn.rb

@@ -204,7 +204,7 @@ def split_me?
     # This method returns true for a column when it requires splitting.
     return false if self.approval_stage < 2
     return false if %w{category text}.include? self.import_data_type
-    self.sheetcells.joins(:category).where(["categories.datagroup_id = ?", self.datagroup_id]).exists?
+    self.sheetcells.where(["category_id is not null and exists(select 1 from categories AS c where c.datagroup_id = ? and c.id = sheetcells.category_id)", self.datagroup_id]).exists?
   end
 
   # users of a datacolumn are those who are responsible for it