Add partial index for deploy keys

Per https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/31384, this
appears to drop the runtime of the query from 1.2 ms (cold), 500ms (warm
cache) to < 20 ms.

db/migrate/20190802235445_add_index_on_id_and_type_and_public_to_keys.rb

+# frozen_string_literal: true
+
+class AddIndexOnIdAndTypeAndPublicToKeys < ActiveRecord::Migration[5.2]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  INDEX_NAME = "index_on_deploy_keys_id_and_type_and_public"
+
+  def up
+    add_concurrent_index(:keys,
+                         [:id, :type],
+                         where: "public = 't'",
+                         unique: true,
+                         name: INDEX_NAME)
+  end
+
+  def down
+    remove_concurrent_index_by_name(:keys, 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: 2019_08_02_012622) do
+ActiveRecord::Schema.define(version: 2019_08_02_235445) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "pg_trgm"
@@ -1770,6 +1770,7 @@ ActiveRecord::Schema.define(version: 2019_08_02_012622) do
     t.boolean "public", default: false, null: false
     t.datetime "last_used_at"
     t.index ["fingerprint"], name: "index_keys_on_fingerprint", unique: true
+    t.index ["id", "type"], name: "index_on_deploy_keys_id_and_type_and_public", unique: true, where: "(public = true)"
     t.index ["user_id"], name: "index_keys_on_user_id"
   end