Replace index on ci_triggers

Relates to https://gitlab.com/gitlab-org/gitlab/issues/34188

changelogs/unreleased/ab-replace-index.yml

+---
+title: Replace index on ci_triggers
+merge_request: 18652
+author:
+type: performance

db/migrate/20191016072826_replace_ci_trigger_requests_index.rb

+# frozen_string_literal: true
+
+class ReplaceCiTriggerRequestsIndex < ActiveRecord::Migration[5.2]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def up
+    add_concurrent_index :ci_trigger_requests, [:trigger_id, :id], order: { id: :desc }
+
+    remove_concurrent_index :ci_trigger_requests, [:trigger_id]
+  end
+
+  def down
+    add_concurrent_index :ci_trigger_requests, [:trigger_id]
+
+    remove_concurrent_index :ci_trigger_requests, [:trigger_id, :id], order: { id: :desc }
+  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_10_04_133612) do
+ActiveRecord::Schema.define(version: 2019_10_16_072826) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "pg_trgm"
@@ -906,7 +906,7 @@ ActiveRecord::Schema.define(version: 2019_10_04_133612) do
     t.datetime "updated_at"
     t.integer "commit_id"
     t.index ["commit_id"], name: "index_ci_trigger_requests_on_commit_id"
-    t.index ["trigger_id"], name: "index_ci_trigger_requests_on_trigger_id"
+    t.index ["trigger_id", "id"], name: "index_ci_trigger_requests_on_trigger_id_and_id", order: { id: :desc }
   end
 
   create_table "ci_triggers", id: :serial, force: :cascade do |t|