Replace events index with partial one

This improves the index size by removing all records with `group_id`
from the index. On GitLab.com this means reducing the size from 9 GB to
8kb (as long as the column stays mostly null).

Relates to
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/32826#note_214690051

changelogs/unreleased/ab-partial-events-index.yml

+---
+title: Replace events index with partial one
+merge_request: 32874
+author:
+type: performance

db/migrate/20190910103144_replace_events_index_on_group_id_with_partial_index.rb

+# frozen_string_literal: true
+
+class ReplaceEventsIndexOnGroupIdWithPartialIndex < ActiveRecord::Migration[5.2]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def up
+    add_concurrent_index(:events, :group_id, where: 'group_id IS NOT NULL', name: 'index_events_on_group_id_partial')
+    remove_concurrent_index_by_name(:events, 'index_events_on_group_id')
+  end
+
+  def down
+    add_concurrent_index(:events, :group_id, name: 'index_events_on_group_id')
+    remove_concurrent_index_by_name(:events, 'index_events_on_group_id_partial')
+  end
+end

db/schema.rb

@@ -1330,7 +1330,7 @@ ActiveRecord::Schema.define(version: 2019_09_12_061145) do
     t.index ["action"], name: "index_events_on_action"
     t.index ["author_id", "project_id"], name: "index_events_on_author_id_and_project_id"
     t.index ["created_at", "author_id"], name: "analytics_index_events_on_created_at_and_author_id"
-    t.index ["group_id"], name: "index_events_on_group_id"
+    t.index ["group_id"], name: "index_events_on_group_id_partial", where: "(group_id IS NOT NULL)"
     t.index ["project_id", "created_at"], name: "index_events_on_project_id_and_created_at"
     t.index ["project_id", "id"], name: "index_events_on_project_id_and_id"
     t.index ["target_type", "target_id"], name: "index_events_on_target_type_and_target_id"