Add index on runner_type for ci_runners

We use this to filter runner and will use this increasingly in future
instead of is_shared so it should be indexed

changelogs/unreleased/46010-add-index-to-runner-type.yml

+---
+title: Add index on runner_type for ci_runners
+merge_request: 18897
+author:
+type: performance

db/migrate/20180511090724_add_index_on_ci_runners_runner_type.rb

+class AddIndexOnCiRunnersRunnerType < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def up
+    add_concurrent_index :ci_runners, :runner_type
+  end
+
+  def down
+    remove_index :ci_runners, :runner_type
+  end
+end

db/schema.rb

@@ -11,7 +11,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
-ActiveRecord::Schema.define(version: 20180508135515) do
+ActiveRecord::Schema.define(version: 20180511090724) do
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -503,6 +503,7 @@ ActiveRecord::Schema.define(version: 20180508135515) do
   add_index "ci_runners", ["contacted_at"], name: "index_ci_runners_on_contacted_at", using: :btree
   add_index "ci_runners", ["is_shared"], name: "index_ci_runners_on_is_shared", using: :btree
   add_index "ci_runners", ["locked"], name: "index_ci_runners_on_locked", using: :btree
+  add_index "ci_runners", ["runner_type"], name: "index_ci_runners_on_runner_type", using: :btree
   add_index "ci_runners", ["token"], name: "index_ci_runners_on_token", using: :btree
   create_table "ci_stages", force: :cascade do |t|