From 3883bc05d26d930689115c6b5d8750c594420615 Mon Sep 17 00:00:00 2001
From: Yorick Peterse <yorickpeterse@gmail.com>
Date: Wed, 8 Jun 2016 12:27:31 +0200
Subject: [PATCH] Remove notification settings in batches

This improves performance of the duplicate notification settings
migration by removing duplicates in batches instead of using one big
"DELETE FROM" query.

The previous query would locally run over 45 minutes without even
finishing. This new setup finished in a matter of seconds.

Fixes #18289
---
 ...remove_duplicated_notification_settings.rb | 31 +++++++++++++++++--
 db/schema.rb                                  |  4 ++-
 2 files changed, 31 insertions(+), 4 deletions(-)

diff --git a/db/migrate/20160603180330_remove_duplicated_notification_settings.rb b/db/migrate/20160603180330_remove_duplicated_notification_settings.rb
index c2fcac4c53..fe1c863b5b 100644
--- a/db/migrate/20160603180330_remove_duplicated_notification_settings.rb
+++ b/db/migrate/20160603180330_remove_duplicated_notification_settings.rb
@@ -1,7 +1,32 @@
 class RemoveDuplicatedNotificationSettings < ActiveRecord::Migration
   def up
-    execute <<-SQL
-      DELETE FROM notification_settings WHERE id NOT IN ( SELECT min_id from (SELECT MIN(id) as min_id FROM notification_settings GROUP BY user_id, source_type, source_id) as dups )
-    SQL
+    duplicates = exec_query(%Q{
+      SELECT user_id, source_type, source_id
+      FROM notification_settings
+      GROUP BY user_id, source_type, source_id
+      HAVING COUNT(*) > 1
+    })
+
+    duplicates.each do |row|
+      uid = row['user_id']
+      stype = connection.quote(row['source_type'])
+      sid = row['source_id']
+
+      execute(%Q{
+        DELETE FROM notification_settings
+        WHERE user_id = #{uid}
+        AND source_type = #{stype}
+        AND source_id = #{sid}
+        AND id != (
+          SELECT id FROM (
+            SELECT min(id) AS id
+            FROM notification_settings
+            WHERE user_id = #{uid}
+            AND source_type = #{stype}
+            AND source_id = #{sid}
+          ) min_ids
+        )
+      })
+    end
   end
 end
diff --git a/db/schema.rb b/db/schema.rb
index 69e37470de..00829d63b6 100644
--- a/db/schema.rb
+++ b/db/schema.rb
@@ -11,7 +11,8 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 20160530150109) do
+ActiveRecord::Schema.define(version: 20160603182247) do
+
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
   enable_extension "pg_trgm"
@@ -676,6 +677,7 @@ ActiveRecord::Schema.define(version: 20160530150109) do
   end
 
   add_index "notification_settings", ["source_id", "source_type"], name: "index_notification_settings_on_source_id_and_source_type", using: :btree
+  add_index "notification_settings", ["user_id", "source_id", "source_type"], name: "index_notifications_on_user_id_and_source_id_and_source_type", unique: true, using: :btree
   add_index "notification_settings", ["user_id"], name: "index_notification_settings_on_user_id", using: :btree
 
   create_table "oauth_access_grants", force: :cascade do |t|