Add index to audit_events to prevent timeouts

Add index on audit_events (entity_id, entity_type, id desc), to
replace existing index on (entity_id, entity_type)

changelogs/unreleased/39474-unable-to-view-project-audit-events-statement-timeouts.yml

+---
+title: Add index to audit_events (entity_id, entity_type, id)
+merge_request: 23998
+author:
+type: performance

db/migrate/20200129172428_add_index_on_audit_events_id_desc.rb

+# frozen_string_literal: true
+
+class AddIndexOnAuditEventsIdDesc < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+  OLD_INDEX_NAME = 'index_audit_events_on_entity_id_and_entity_type'
+  NEW_INDEX_NAME = 'index_audit_events_on_entity_id_and_entity_type_and_id_desc'
+
+  disable_ddl_transaction!
+
+  def up
+    add_concurrent_index :audit_events, [:entity_id, :entity_type, :id], name: NEW_INDEX_NAME,
+      order: { entity_id: :asc, entity_type: :asc, id: :desc }
+
+    remove_concurrent_index_by_name :audit_events, OLD_INDEX_NAME
+  end
+
+  def down
+    add_concurrent_index :audit_events, [:entity_id, :entity_type], name: OLD_INDEX_NAME
+
+    remove_concurrent_index_by_name :audit_events, NEW_INDEX_NAME
+  end
+end

db/schema.rb

@@ -465,7 +465,7 @@ ActiveRecord::Schema.define(version: 2020_01_30_161817) do
     t.datetime "created_at"
     t.datetime "updated_at"
     t.index ["created_at", "author_id"], name: "analytics_index_audit_events_on_created_at_and_author_id"
-    t.index ["entity_id", "entity_type"], name: "index_audit_events_on_entity_id_and_entity_type"
+    t.index ["entity_id", "entity_type", "id"], name: "index_audit_events_on_entity_id_and_entity_type_and_id_desc", order: { id: :desc }
   end
 
   create_table "award_emoji", id: :serial, force: :cascade do |t|