Optimizes the service desk issue counter queries

Removes distinct and adds an index for batch counting in usage data

db/migrate/20200306170211_add_index_on_author_id_and_id_and_created_at_to_issues.rb

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

db/schema.rb

@@ -2206,6 +2206,7 @@ ActiveRecord::Schema.define(version: 2020_03_06_170531) do
     t.integer "duplicated_to_id"
     t.integer "promoted_to_epic_id"
     t.integer "health_status", limit: 2
+    t.index ["author_id", "id", "created_at"], name: "index_issues_on_author_id_and_id_and_created_at"
     t.index ["author_id"], name: "index_issues_on_author_id"
     t.index ["closed_by_id"], name: "index_issues_on_closed_by_id"
     t.index ["confidential"], name: "index_issues_on_confidential"

ee/changelogs/unreleased/208888-optimize-service_desk_issue-counters-related-to-the-issue.yml

+---
+title: Optimize service desk issue counters query performance in usage data
+merge_request: 26445
+author:
+type: performance

ee/lib/ee/gitlab/usage_data.rb

@@ -281,7 +281,7 @@ module EE
             projects_jira_dvcs_cloud_active: distinct_count(::Project.with_active_jira_services.with_jira_dvcs_cloud.where(time_period), :creator_id),
             projects_jira_dvcs_server_active: distinct_count(::Project.with_active_jira_services.with_jira_dvcs_server.where(time_period), :creator_id),
             service_desk_enabled_projects: distinct_count(::Project.with_active_services.service_desk_enabled.where(time_period), :creator_id),
-            service_desk_issues: distinct_count(::Issue.service_desk.where(time_period)),
+            service_desk_issues: count(::Issue.service_desk.where(time_period)),
             todos: distinct_count(::Todo.where(time_period), :author_id)
           }
         end