modqueue: fix performance regression from including appeals.

* Add index on posts.is_deleted. The modqueue was slow because we the
 appeal condition wasn't constrained to deleted posts, so it degraded to
 a full table scan.

* Avoid extra queries for calculating the page count and disapproval counts.

app/controllers/modqueue_controller.rb

@@ -5,12 +5,12 @@ class ModqueueController < ApplicationController
   def index
     authorize :modqueue
     @posts = Post.includes(:appeals, :disapprovals, :uploader, flags: [:creator]).in_modqueue.available_for_moderation(CurrentUser.user, hidden: search_params[:hidden])
-    @posts = @posts.paginated_search(params, order: "modqueue", count_pages: true)
-
     @modqueue_posts = @posts.reselect(nil).reorder(nil).offset(nil).limit(nil)
+    @posts = @posts.paginated_search(params, order: "modqueue", count_pages: true, count: @modqueue_posts.to_a.size)
+
     @pending_post_count = @modqueue_posts.select(&:is_pending?).count
     @flagged_post_count = @modqueue_posts.select(&:is_flagged?).count
-    @disapproval_reasons = PostDisapproval.where(post: @modqueue_posts.reselect(:id)).where.not(reason: "disinterest").group(:reason).order(count: :desc).distinct.count(:post_id)
+    @disapproval_reasons = PostDisapproval.where(post_id: @modqueue_posts.map(&:id)).where.not(reason: "disinterest").group(:reason).order(count: :desc).distinct.count(:post_id)
     @uploaders = @modqueue_posts.map(&:uploader).tally.sort_by(&:last).reverse.take(20).to_h
 
     @tags = RelatedTagCalculator.frequent_tags_for_post_relation(@modqueue_posts)

app/models/post.rb

@@ -63,7 +63,7 @@ class Post < ApplicationRecord
   scope :flagged, -> { where(is_flagged: true) }
   scope :banned, -> { where(is_banned: true) }
   scope :active, -> { where(is_pending: false, is_deleted: false, is_flagged: false).where.not(id: PostAppeal.pending) }
-  scope :appealed, -> { where(id: PostAppeal.pending.select(:post_id)) }
+  scope :appealed, -> { deleted.where(id: PostAppeal.pending.select(:post_id)) }
   scope :in_modqueue, -> { pending.or(flagged).or(appealed) }
   scope :expired, -> { pending.where("posts.created_at < ?", Danbooru.config.moderation_period.ago) }
 

db/migrate/20200816175151_add_deleted_index_on_posts.rb

@@ -0,0 +1,5 @@
+class AddDeletedIndexOnPosts < ActiveRecord::Migration[6.0]
+  def change
+    add_index :posts, :is_deleted, where: "is_deleted = true"
+  end
+end

db/structure.sql

@@ -6757,6 +6757,13 @@ CREATE INDEX index_posts_on_image_height ON public.posts USING btree (image_heig
 CREATE INDEX index_posts_on_image_width ON public.posts USING btree (image_width);
 
 
+--
+-- Name: index_posts_on_is_deleted; Type: INDEX; Schema: public; Owner: -
+--
+
+CREATE INDEX index_posts_on_is_deleted ON public.posts USING btree (is_deleted) WHERE (is_deleted = true);
+
+
 --
 -- Name: index_posts_on_is_flagged; Type: INDEX; Schema: public; Owner: -
 --
@@ -7410,6 +7417,8 @@ INSERT INTO "schema_migrations" (version) VALUES
 ('20200403210353'),
 ('20200406054838'),
 ('20200427190519'),
-('20200803022359');
+('20200520060951'),
+('20200803022359'),
+('20200816175151');