From aa2a9da4b2e0f6b98963144888bb0a6ed9b71063 Mon Sep 17 00:00:00 2001
From: Sam Saffron <sam.saffron@gmail.com>
Date: Tue, 12 May 2020 09:51:59 +1000
Subject: [PATCH] PERF: reduce exclusive locking when changing tables

posts and topics tables can be huge, this migration change
ensures the migration performs less work in one giant transaction
reducing risk of deadlocks

Migration is re-runnable thanks to NOT EXISTS checks
---
 ...9095034_add_topic_thumbnail_information.rb | 37 ++++++++++++++-----
 1 file changed, 27 insertions(+), 10 deletions(-)

diff --git a/db/migrate/20200429095034_add_topic_thumbnail_information.rb b/db/migrate/20200429095034_add_topic_thumbnail_information.rb
index be34f93c32..ee48265c6d 100644
--- a/db/migrate/20200429095034_add_topic_thumbnail_information.rb
+++ b/db/migrate/20200429095034_add_topic_thumbnail_information.rb
@@ -1,19 +1,36 @@
 # frozen_string_literal: true
 
 class AddTopicThumbnailInformation < ActiveRecord::Migration[6.0]
-  def change
-    add_reference :posts, :image_upload
-    add_reference :topics, :image_upload
+  disable_ddl_transaction!
 
-    add_column :theme_modifier_sets, :topic_thumbnail_sizes, :string, array: true
+  def up
 
-    create_table :topic_thumbnails do |t|
-      t.references :upload, null: false
-      t.references :optimized_image, null: true
-      t.integer :max_width, null: false
-      t.integer :max_height, null: false
+    # tables are huge ... avoid holding on to large number of locks by doing one at a time
+    execute <<~SQL
+      ALTER TABLE posts
+      ADD COLUMN IF NOT EXISTS image_upload_id bigint
+    SQL
+
+    execute <<~SQL
+      ALTER TABLE topics
+      ADD COLUMN IF NOT EXISTS image_upload_id bigint
+    SQL
+
+    ActiveRecord::Base.transaction do
+      add_column :theme_modifier_sets, :topic_thumbnail_sizes, :string, array: true
+
+      create_table :topic_thumbnails do |t|
+        t.references :upload, null: false
+        t.references :optimized_image, null: true
+        t.integer :max_width, null: false
+        t.integer :max_height, null: false
+      end
+
+      add_index :topic_thumbnails, [:upload_id, :max_width, :max_height], name: :unique_topic_thumbnails, unique: true
     end
+  end
 
-    add_index :topic_thumbnails, [:upload_id, :max_width, :max_height], name: :unique_topic_thumbnails, unique: true
+  def down
+    raise IrriversibleMigration
   end
 end