Create index for users.unconfirmed_emailab/unconfirmed-email-index

This speeds up the following query:

```sql
SELECT  users.* FROM users WHERE users.unconfirmed_email = ? ORDER BY
users.id ASC LIMIT 1
```

Presumably, this is a query coming from Devise.

Context is https://gitlab.com/gitlab-org/gitlab-ce/issues/66958.

1 files changed, 17 insertions, 0 deletions

diff --git a/db/migrate/20190904173203_add_index_on_users_unconfirmed_email.rb b/db/migrate/20190904173203_add_index_on_users_unconfirmed_email.rb
new file mode 100644
index 00000000000..e78d47f023f
--- /dev/null
+++ b/db/migrate/20190904173203_add_index_on_users_unconfirmed_email.rb
@@ -0,0 +1,17 @@
+# frozen_string_literal: true
+
+class AddIndexOnUsersUnconfirmedEmail < ActiveRecord::Migration[5.2]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def up
+    add_concurrent_index :users, :unconfirmed_email, where: 'unconfirmed_email IS NOT NULL'
+  end
+
+  def down
+    remove_concurrent_index :users, :unconfirmed_email, where: 'unconfirmed_email IS NOT NULL'
+  end
+end