Add index on public_emails for users table

Clean up issue-62684

Update issue-62684 changelog

Update down migrate for issue-62684

changelogs/unreleased/62684-add-index-public-email-on-users.yml

+---
+title: Add index on public_email for users
+merge_request: 29430
+author:
+type: performance

db/migrate/20190607190856_add_index_to_users_public_emails.rb

+# frozen_string_literal: true
+
+# See http://doc.gitlab.com/ce/development/migration_style_guide.html
+# for more information on how to write migrations for GitLab.
+
+class AddIndexToUsersPublicEmails < ActiveRecord::Migration[5.1]
+  include Gitlab::Database::MigrationHelpers
+
+  # Set this constant to true if this migration requires downtime.
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def up
+    add_concurrent_index :users, [:public_email],
+                         where: "public_email != ''"
+  end
+
+  def down
+    remove_concurrent_index :users, [:public_email],
+                         where: "public_email != ''"
+  end
+end

db/schema.rb

@@ -2388,6 +2388,7 @@ ActiveRecord::Schema.define(version: 20190611161641) do
     t.index ["incoming_email_token"], name: "index_users_on_incoming_email_token", using: :btree
     t.index ["name"], name: "index_users_on_name", using: :btree
     t.index ["name"], name: "index_users_on_name_trigram", using: :gin, opclasses: {"name"=>"gin_trgm_ops"}
+    t.index ["public_email"], name: "index_users_on_public_email", where: "((public_email)::text <> ''::text)", using: :btree
     t.index ["reset_password_token"], name: "index_users_on_reset_password_token", unique: true, using: :btree
     t.index ["state"], name: "index_users_on_state", using: :btree
     t.index ["username"], name: "index_users_on_username", using: :btree