Add system check for missing database indexes (#30888)

spec/lib/admin/db/schema_parser_spec.rb

@@ -0,0 +1,49 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+describe Admin::Db::SchemaParser do
+  let(:dummy_schema) do
+    <<~SCHEMA
+      # Comment
+      ActiveRecord::Schema[7.1].define(version: 23) do
+        create_table "people", force: :cascade do |t|
+          t.string "name"
+        end
+
+        create_table "posts", force: :cascade do |t|
+          t.string "title", null: false
+          t.bigint "size", null: false
+          t.string "description"
+          # t.index ["size", "title"], name: "index_posts_on_size_and_title"
+          t.index ["title"], name: "index_posts_on_title", unique: true
+          t.index ["size"], name: "index_posts_on_size"
+        end
+
+        # add_index "people", ["name"], name: "commented_out_index"
+        add_index "people", ["name"], name: "index_people_on_name"
+      end
+    SCHEMA
+  end
+  let(:schema_parser) { described_class.new(dummy_schema) }
+
+  describe '#indexes_by_table' do
+    subject { schema_parser.indexes_by_table }
+
+    it 'returns index info for all affected tables' do
+      expect(subject.keys).to match_array(%w(people posts))
+    end
+
+    it 'returns all index information for the `people` table' do
+      people_info = subject['people']
+      expect(people_info.map(&:name)).to contain_exactly('index_people_on_name')
+    end
+
+    it 'returns all index information for the `posts` table' do
+      posts_info = subject['posts']
+      expect(posts_info.map(&:name)).to contain_exactly(
+        'index_posts_on_title', 'index_posts_on_size'
+      )
+    end
+  end
+end