PERF: improve performance of category topic list

In some cases CTE caused pathologically bad query plans.
This optimises it so query runs by itself and caches for lifetime
of the topic query object.

This lightweight caching is done cause topic query will often
execute two queries (one for pinned and one for non pinned)

lib/topic_query.rb

@@ -669,20 +669,11 @@ class TopicQuery
       if options[:no_subcategories]
         result = result.where('categories.id = ?', category_id)
       else
-        sql = <<~SQL
-            categories.id IN (
-              WITH RECURSIVE subcategories AS (
-                SELECT :category_id id, 1 depth
-                UNION
-                SELECT categories.id, (subcategories.depth + 1) depth
-                FROM categories
-                JOIN subcategories ON subcategories.id = categories.parent_category_id
-                WHERE subcategories.depth < :max_category_nesting
-            )
-              SELECT subcategories.id FROM subcategories
-            ) AND (categories.id = :category_id OR topics.id != categories.topic_id)
+        result = result.where(<<~SQL, subcategory_ids: subcategory_ids(category_id), category_id: category_id)
+          categories.id in (:subcategory_ids) AND (
+            categories.topic_id <> topics.id OR categories.id = :category_id
+          )
           SQL
-        result = result.where(sql, category_id: category_id, max_category_nesting: SiteSetting.max_category_nesting)
       end
       result = result.references(:categories)
 
@@ -1061,6 +1052,29 @@ class TopicQuery
 
   private
 
+  def subcategory_ids(category_id)
+    @subcategory_ids ||= {}
+    @subcategory_ids[category_id] ||=
+      begin
+        sql = <<~SQL
+            WITH RECURSIVE subcategories AS (
+                SELECT :category_id id, 1 depth
+                UNION
+                SELECT categories.id, (subcategories.depth + 1) depth
+                FROM categories
+                JOIN subcategories ON subcategories.id = categories.parent_category_id
+                WHERE subcategories.depth < :max_category_nesting
+            )
+            SELECT id FROM subcategories
+          SQL
+        DB.query_single(
+          sql,
+          category_id: category_id,
+          max_category_nesting: SiteSetting.max_category_nesting
+        )
+      end
+  end
+
   def sanitize_sql_array(input)
     ActiveRecord::Base.public_send(:sanitize_sql_array, input.join(','))
   end