From b4999acadd3783e0953f04729cafc2fe0598db87 Mon Sep 17 00:00:00 2001
From: Sam Saffron <sam.saffron@gmail.com>
Date: Sat, 29 Feb 2020 15:40:54 +1100
Subject: [PATCH] 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 | 40 +++++++++++++++++++++++++++-------------
 1 file changed, 27 insertions(+), 13 deletions(-)

diff --git a/lib/topic_query.rb b/lib/topic_query.rb
index bfb787ce9f1..f77d37e7464 100644
--- a/lib/topic_query.rb
+++ b/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