Improve performance of User.by_login

Performance is improved in two steps:

1. On PostgreSQL an expression index is used for checking lower(email)
   and lower(username).
2. The check to determine if we're searching for a username or Email is
   moved to Ruby. Thanks to @haynes for suggesting and writing the
   initial implementation of this.

Moving the check to Ruby makes this method an additional 1.5 times
faster compared to doing the check in the SQL query.

With performance being improved I've now also tweaked the amount of
iterations required by the User.by_login benchmark. This method now runs
between 900 and 1000 iterations per second.

CHANGELOG

@@ -2,6 +2,7 @@ Please view this file on the master branch, on stable branches it's out of date.
 v 8.1.0 (unreleased)
   - Make diff file view easier to use on mobile screens (Stan Hu)
+  - Improved performance of finding users by username or Email address
   - Add support for creating directories from Files page (Stan Hu)
   - Allow removing of project without confirmation when JavaScript is disabled (Stan Hu)
   - Support filtering by "Any" milestone or issue and fix "No Milestone" and "No Label" filters (Stan Hu)

app/models/user.rb

@@ -68,6 +68,7 @@ class User < ActiveRecord::Base
   include Referable
   include Sortable
   include TokenAuthenticatable
+  include CaseSensitivity
   default_value_for :admin, false
   default_value_for :can_create_group, gitlab_config.default_can_create_group
@@ -273,8 +274,13 @@ class User < ActiveRecord::Base
     end
     def by_login(login)
-      where('lower(username) = :value OR lower(email) = :value',
-            value: login.to_s.downcase).first
+      return nil unless login
+
+      if login.include?('@'.freeze)
+        unscoped.iwhere(email: login).take
+      else
+        unscoped.iwhere(username: login).take
+      end
     end
     def find_by_username!(username)

db/migrate/20151008110232_add_users_lower_username_email_indexes.rb

+class AddUsersLowerUsernameEmailIndexes < ActiveRecord::Migration
+  disable_ddl_transaction!
+
+  def up
+    return unless Gitlab::Database.postgresql?
+
+    execute 'CREATE INDEX CONCURRENTLY index_on_users_lower_username ON users (LOWER(username));'
+    execute 'CREATE INDEX CONCURRENTLY index_on_users_lower_email ON users (LOWER(email));'
+  end
+
+  def down
+    return unless Gitlab::Database.postgresql?
+
+    remove_index :users, :index_on_users_lower_username
+    remove_index :users, :index_on_users_lower_email
+  end
+end

lib/tasks/migrate/setup_postgresql.rake

 require Rails.root.join('db/migrate/20151007120511_namespaces_projects_path_lower_indexes')
+require Rails.root.join('db/migrate/20151008110232_add_users_lower_username_email_indexes')
 desc 'GitLab | Sets up PostgreSQL'
 task setup_postgresql: :environment do
   NamespacesProjectsPathLowerIndexes.new.up
+  AddUsersLowerUsernameEmailIndexes.new.up
 end

spec/benchmarks/models/user_spec.rb

@@ -11,7 +11,9 @@ describe User, benchmark: true do
       end
     end
-    let(:iterations) { 1000 }
+    # The iteration count is based on the query taking little over 1 ms when
+    # using PostgreSQL.
+    let(:iterations) { 900 }
     describe 'using a capitalized username' do
       benchmark_subject { User.by_login('Alice') }