Reduce SQL queries to find a unsolved recommended RfC

Fixes CODEOCEAN-JQ

app/models/submission.rb

@@ -133,9 +133,14 @@ class Submission < ApplicationRecord
   end
 
   def unsolved_rfc(user = self.user)
-    Pundit.policy_scope(user, RequestForComment).unsolved.where(exercise_id: exercise).where.not(question: nil).where(created_at: OLDEST_RFC_TO_SHOW.ago..Time.current).order('RANDOM()').find do |rfc_element|
-      ((rfc_element.comments_count < MAX_COMMENTS_ON_RECOMMENDED_RFC) && !rfc_element.question.empty?)
-    end
+    Pundit.policy_scope(user, RequestForComment)
+      .unsolved.where.not(question: [nil, ''])
+      .where(exercise_id: exercise, created_at: OLDEST_RFC_TO_SHOW.ago...)
+      .left_joins(:comments)
+      .having('COUNT(comments.id) < ?', MAX_COMMENTS_ON_RECOMMENDED_RFC)
+      .group(:id)
+      .order('RANDOM()').limit(1)
+      .first
   end
 
   # @raise [Runner::Error] if the score could not be calculated due to a failure with the runner.

db/migrate/20230314084733_add_index_for_recommended_rfcs.rb

@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+class AddIndexForRecommendedRfcs < ActiveRecord::Migration[7.0]
+  def change
+    add_index :request_for_comments, %i[exercise_id created_at], where: "(NOT solved OR solved IS NULL) AND (question IS NOT NULL AND question <> '')", name: :index_unresolved_recommended_rfcs
+  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[7.0].define(version: 2023_02_19_113125) do
+ActiveRecord::Schema[7.0].define(version: 2023_03_14_084733) do
   # These are extensions that must be enabled in order to support this database
   enable_extension "pg_trgm"
   enable_extension "pgcrypto"
@@ -387,6 +387,7 @@ ActiveRecord::Schema[7.0].define(version: 2023_02_19_113125) do
     t.text "thank_you_note"
     t.boolean "full_score_reached", default: false
     t.integer "times_featured", default: 0
+    t.index ["exercise_id", "created_at"], name: "index_unresolved_recommended_rfcs", where: "(((NOT solved) OR (solved IS NULL)) AND ((question IS NOT NULL) AND (question <> ''::text)))"
     t.index ["exercise_id"], name: "index_request_for_comments_on_exercise_id"
     t.index ["submission_id"], name: "index_request_for_comments_on_submission_id"
     t.index ["user_id", "user_type", "created_at"], name: "index_rfc_on_user_and_created_at", order: { created_at: :desc }
@@ -570,6 +571,25 @@ ActiveRecord::Schema[7.0].define(version: 2023_02_19_113125) do
     t.index ["user_type", "user_id"], name: "index_user_proxy_exercise_exercises_on_user"
   end
 
+  create_table "wk2020_until_rfc_reply", id: false, force: :cascade do |t|
+    t.integer "user_id"
+    t.integer "exercise_id"
+    t.interval "working_time_until_rfc_reply"
+  end
+
+  create_table "wk2020_with_wk_until_rfc", id: false, force: :cascade do |t|
+    t.string "external_user_id", limit: 255
+    t.integer "user_id"
+    t.integer "exercise_id"
+    t.float "max_score"
+    t.float "max_reachable_points"
+    t.interval "working_time"
+    t.interval "working_time_until_rfc"
+    t.interval "working_time_until_rfc_reply"
+    t.time "percentile75"
+    t.time "percentile90"
+  end
+
   add_foreign_key "authentication_tokens", "study_groups"
   add_foreign_key "community_solution_contributions", "community_solution_locks"
   add_foreign_key "community_solution_contributions", "community_solutions"