Remove ORDER BY clause from COUNT queries

Since we just used the pre-assembled relations to then call `#size` on
them, this caused queries like this to be sent to the database:
```
SELECT COUNT(*) FROM (
  SELECT 1 AS one
  FROM `posts`
  ORDER BY `posts`.`author` DESC, `posts`.`id` DESC
  LIMIT 3
) subquery_for_count
```

The `ORDER BY` part will never influence the number of returned records.
But depending on the used database and their query optimization
settings, it will make the query less performant.

Therefore, remove any order from the queries before calling `#size` on
them. With `ActiveRecord` this can be achieved by calling `#reorder` on
the relation with an empty string.

CHANGELOG.md

@@ -16,6 +16,7 @@ These are the latest changes on the project's `master` branch that have not yet
 
 ### Changed
 - **Breaking change:** The way records are retrieved from a given cursor has been changed to no longer use `CONCAT` but instead simply use a compound `WHERE` clause in case of a custom order and having both the custom field as well as the `id` field in the `ORDER BY` query. This is a breaking change since it now changes the internal order of how records with the same value of the `order_by` field are returned.
+- Remove `ORDER BY` clause from `COUNT` queries
 
 ## [0.1.3] - 2021-03-17
 

lib/rails_cursor_pagination/paginator.rb

@@ -159,7 +159,7 @@ def page
     #
     # @return [Integer]
     def total
-      memoize(:total) { @relation.size }
+      memoize(:total) { @relation.reorder('').size }
     end
 
     # Check if the pagination direction is forward
@@ -186,11 +186,12 @@ def previous_page?
         # When paginating forward, we can only have a previous page if we were
         # provided with a cursor and there were records discarded after applying
         # this filter. These records would have to be on previous pages.
-        @cursor.present? && filtered_and_sorted_relation.size < total
+        @cursor.present? &&
+          filtered_and_sorted_relation.reorder('').size < total
       else
         # When paginating backwards, if we managed to load one more record than
         # requested, this record will be available on the previous page.
-        @page_size < limited_relation_plus_one.size
+        @page_size < limited_relation_plus_one.reorder('').size
       end
     end
 
@@ -201,12 +202,12 @@ def next_page?
       if paginate_forward?
         # When paginating forward, if we managed to load one more record than
         # requested, this record will be available on the next page.
-        @page_size < limited_relation_plus_one.size
+        @page_size < limited_relation_plus_one.reorder('').size
       else
         # When paginating backward, if applying our cursor reduced the number
         # records returned, we know that the missing records will be on
         # subsequent pages.
-        filtered_and_sorted_relation.size < total
+        filtered_and_sorted_relation.reorder('').size < total
       end
     end