favorites: merge favorites subtables.

Merge the 100 favorite subtables into a single table.

Previously the favorites table was partitioned by user id into 100
subtables to try to make searching by user id faster. This wasn't really
necessary and probably slower than just making an index on
(favorites.user_id, favorites.id) to satisfy ordfav searches. BTree
indexes are logarithmic so dividing an index by 100 doesn't make it 100
times faster to search; instead it just removes a layer or two from the
tree.

This also adds a uniqueness index on (user_id, post_id) to prevent
duplicate favorites. Previously we had to check for duplicates at the
application layer, which required careful locking to do it correctly.

Finally, this adds an index on favorites.id, which was surprisingly
missing before. This made ordering and deleting favorites by id really
slow because it degraded to a sequential scan.

app/logical/post_query_builder.rb

@@ -431,8 +431,7 @@ class PostQueryBuilder
     favuser = User.find_by_name(username)
 
     if favuser.present? && Pundit.policy!(current_user, favuser).can_see_favorites?
-      favorites = Favorite.from("favorites_#{favuser.id % 100} AS favorites").where(user: favuser)
-      Post.where(id: favorites.select(:post_id))
+      Post.where(id: favuser.favorites.select(:post_id))
     else
       Post.none
     end
@@ -509,6 +508,8 @@ class PostQueryBuilder
       relation = search_order(relation, "created_at_desc")
     elsif find_metatag(:order) == "custom"
       relation = search_order_custom(relation, select_metatags(:id).map(&:value))
+    elsif has_metatag?(:ordfav)
+      # no-op
     else
       relation = search_order(relation, find_metatag(:order))
     end

app/models/favorite.rb

@@ -4,7 +4,7 @@ class Favorite < ApplicationRecord
   belongs_to :post
   belongs_to :user
 
-  scope :for_user, ->(user_id) { where("favorites.user_id % 100 = ? AND favorites.user_id = ?", user_id.to_i % 100, user_id) }
+  scope :for_user, ->(user_id) { where(user_id: user_id) }
   scope :public_favorites, -> { where(user: User.bit_prefs_match(:enable_private_favorites, false)) }
 
   def self.visible(user)

app/models/user.rb

@@ -138,7 +138,7 @@ class User < ApplicationRecord
   has_many :forum_posts, -> {order("forum_posts.created_at, forum_posts.id")}, :foreign_key => "creator_id"
   has_many :user_name_change_requests, -> {order("user_name_change_requests.created_at desc")}
   has_many :favorite_groups, -> {order(name: :asc)}, foreign_key: :creator_id
-  has_many :favorites, ->(rec) {where("user_id % 100 = #{rec.id % 100} and user_id = #{rec.id}").order("id desc")}
+  has_many :favorites
   has_many :ip_bans, foreign_key: :creator_id
   has_many :tag_aliases, foreign_key: :creator_id
   has_many :tag_implications, foreign_key: :creator_id