Optimizes the notes counter query

Adds an index for batch counting in usage data

changelogs/unreleased/208890-optimize-notes-counters.yml

+---
+title: Optimize notes counters in usage data
+merge_request: 26871
+author:
+type: performance

db/migrate/20200310133822_add_index_on_author_id_and_id_and_created_at_to_notes.rb

+# frozen_string_literal: true
+
+class AddIndexOnAuthorIdAndIdAndCreatedAtToNotes < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def up
+    add_concurrent_index :notes, [:author_id, :created_at]
+    remove_concurrent_index :notes, [:author_id]
+  end
+
+  def down
+    add_concurrent_index :notes, [:author_id]
+    remove_concurrent_index :notes, [:author_id, :created_at]
+  end
+end

db/schema.rb

@@ -2826,7 +2826,7 @@ ActiveRecord::Schema.define(version: 2020_03_11_165635) do
     t.boolean "resolved_by_push"
     t.bigint "review_id"
     t.boolean "confidential"
-    t.index ["author_id"], name: "index_notes_on_author_id"
+    t.index ["author_id", "created_at"], name: "index_notes_on_author_id_and_created_at"
     t.index ["commit_id"], name: "index_notes_on_commit_id"
     t.index ["created_at"], name: "index_notes_on_created_at"
     t.index ["discussion_id"], name: "index_notes_on_discussion_id"