Add partial indexes on todos to handle users with many todos

changelogs/unreleased/add-indexes-to-todos-for-heavy-users-like-sean.yml

+---
+title: Add partial indexes on todos to handle users with many todos
+merge_request:
+author:
+type: performance

db/migrate/20180309160427_add_partial_indexes_on_todos.rb

+# See http://doc.gitlab.com/ce/development/migration_style_guide.html
+# for more information on how to write migrations for GitLab.
+
+class AddPartialIndexesOnTodos < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  # Set this constant to true if this migration requires downtime.
+  DOWNTIME = false
+
+   disable_ddl_transaction!
+
+   INDEX_NAME_PENDING="index_todos_on_user_id_and_id_pending"
+   INDEX_NAME_DONE="index_todos_on_user_id_and_id_done"
+   
+  def up
+    unless index_exists?(:todos, [:user_id, :id], name: INDEX_NAME_PENDING)
+      add_concurrent_index(:todos, [:user_id, :id], where: "state='pending'", name: INDEX_NAME_PENDING)
+    end
+    unless index_exists?(:todos, [:user_id, :id], name: INDEX_NAME_DONE)
+      add_concurrent_index(:todos, [:user_id, :id], where: "state='done'", name: INDEX_NAME_DONE)
+    end
+  end
+
+  def down
+    remove_concurrent_index(:todos, [:user_id, :id], where: "state='pending'", name: INDEX_NAME_PENDING)
+    remove_concurrent_index(:todos, [:user_id, :id], where: "state='done'", name: INDEX_NAME_DONE)
+  end    
+end

db/schema.rb

@@ -11,7 +11,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 20180309121820) do
+ActiveRecord::Schema.define(version: 20180309160427) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -1778,6 +1778,8 @@ ActiveRecord::Schema.define(version: 20180309121820) do
   add_index "todos", ["note_id"], name: "index_todos_on_note_id", using: :btree
   add_index "todos", ["project_id"], name: "index_todos_on_project_id", using: :btree
   add_index "todos", ["target_type", "target_id"], name: "index_todos_on_target_type_and_target_id", using: :btree
+  add_index "todos", ["user_id", "id"], name: "index_todos_on_user_id_and_id_done", where: "((state)::text = 'done'::text)", using: :btree
+  add_index "todos", ["user_id", "id"], name: "index_todos_on_user_id_and_id_pending", where: "((state)::text = 'pending'::text)", using: :btree
   add_index "todos", ["user_id"], name: "index_todos_on_user_id", using: :btree
 
   create_table "trending_projects", force: :cascade do |t|