Add index on users.unlock_token

This prevents queries using the unlock_token from bogging down the
server.

changelogs/unreleased/sh-add-index-unlock-token.yml

+---
+title: Add index on users.unlock_token
+merge_request: 276298
+author:
+type: performance

db/migrate/20200319203901_add_index_on_users_unlock_token.rb

+# frozen_string_literal: true
+
+class AddIndexOnUsersUnlockToken < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+  INDEX_NAME = 'index_users_on_unlock_token'
+
+  disable_ddl_transaction!
+
+  def up
+    add_concurrent_index :users, :unlock_token, unique: true, name: INDEX_NAME
+  end
+
+  def down
+    remove_concurrent_index :users, :unlock_token, unique: true, name: INDEX_NAME
+  end
+end

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 2020_03_18_152134) do
+ActiveRecord::Schema.define(version: 2020_03_19_203901) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "pg_trgm"
@@ -4441,6 +4441,7 @@ ActiveRecord::Schema.define(version: 2020_03_18_152134) do
     t.index ["state"], name: "index_users_on_state"
     t.index ["static_object_token"], name: "index_users_on_static_object_token", unique: true
     t.index ["unconfirmed_email"], name: "index_users_on_unconfirmed_email", where: "(unconfirmed_email IS NOT NULL)"
+    t.index ["unlock_token"], name: "index_users_on_unlock_token", unique: true
     t.index ["user_type"], name: "index_users_on_user_type"
     t.index ["username"], name: "index_users_on_username"
     t.index ["username"], name: "index_users_on_username_trigram", opclass: :gin_trgm_ops, using: :gin