Optimizes the todo counter query

adds an index for batch counting in usage data

changelogs/unreleased/208891-optimize-todos-counters.yml

+---
+title: Optimize todos counters in usage data
+merge_request: 26442
+author:
+type: performance

db/migrate/20200306170531_add_index_on_author_id_and_created_at_to_todos.rb

+# frozen_string_literal: true
+class AddIndexOnAuthorIdAndCreatedAtToTodos < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def up
+    add_concurrent_index :todos, [:author_id, :created_at]
+  end
+
+  def down
+    remove_concurrent_index :todos, [:author_id, :created_at]
+  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: 2020_03_04_160823) do
+ActiveRecord::Schema.define(version: 2020_03_06_170531) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "pg_trgm"
@@ -4150,6 +4150,7 @@ ActiveRecord::Schema.define(version: 2020_03_04_160823) do
     t.integer "note_id"
     t.string "commit_id"
     t.integer "group_id"
+    t.index ["author_id", "created_at"], name: "index_todos_on_author_id_and_created_at"
     t.index ["author_id"], name: "index_todos_on_author_id"
     t.index ["commit_id"], name: "index_todos_on_commit_id"
     t.index ["group_id"], name: "index_todos_on_group_id"