Improve performance for occupies:X searches

I noticed in the app that these queries were slowwww! I was able to
track it down to a bad query plan, as we explain in the comment.

I searched online for "mysql query performance filter on one join table
sort by another", and was surprised to find this answer suggest a
subquery, which I've often been told to expect to be slower compared to
joins? But it certainly worked in this case!

https://stackoverflow.com/questions/35679693/mysql-optimize-join-with-filter-and-order-on-different-tables

app/models/item.rb

@@ -60,8 +60,23 @@ class Item < ApplicationRecord
   }
   scope :occupies, ->(zone_label) {
     zone_ids = Zone.matching_label(zone_label).map(&:id)
+
+    # NOTE: In searches, this query performs much better using a subquery
+    # instead of joins! This is because, in the joins case, filtering by an
+    # `swf_assets` field but sorting by an `items` field causes the query
+    # planner to only be able to use an index for *one* of them. In this case,
+    # MySQL can use the `swf_assets`.`zone_id` index to get the item IDs for
+    # the subquery, then use the `items`.`name` index to sort them.
+    i = arel_table
+    psa = ParentSwfAssetRelationship.arel_table
     sa = SwfAsset.arel_table
-    joins(:swf_assets).where(sa[:zone_id].in(zone_ids)).distinct
+    where(
+      ParentSwfAssetRelationship.joins(:swf_asset).
+        where(sa[:zone_id].in(zone_ids)).
+        where(psa[:parent_type].eq("Item")).
+        where(psa[:parent_id].eq(i[:id])).
+        arel.exists
+    )
   }
   scope :not_occupies, ->(zone_label) {
     zone_ids = Zone.matching_label(zone_label).map(&:id)

db/migrate/20240603181855_add_index_on_name_to_items.rb

@@ -0,0 +1,5 @@
+class AddIndexOnNameToItems < ActiveRecord::Migration[7.1]
+  def change
+    add_index :items, :name
+  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[7.1].define(version: 2024_05_22_233638) do
+ActiveRecord::Schema[7.1].define(version: 2024_06_03_181855) do
   create_table "alt_styles", charset: "utf8mb4", collation: "utf8mb4_unicode_520_ci", force: :cascade do |t|
     t.integer "species_id", null: false
     t.integer "color_id", null: false
@@ -140,6 +140,7 @@ ActiveRecord::Schema[7.1].define(version: 2024_05_22_233638) do
     t.index ["modeling_status_hint", "created_at"], name: "items_modeling_status_hint_and_created_at"
     t.index ["modeling_status_hint", "id"], name: "items_modeling_status_hint_and_id"
     t.index ["modeling_status_hint"], name: "items_modeling_status_hint"
+    t.index ["name"], name: "index_items_on_name"
   end
 
   create_table "login_cookies", id: :integer, charset: "utf8mb4", collation: "utf8mb4_unicode_520_ci", force: :cascade do |t|