Chore: Speed up admin learner search (#4918)

Because:
- The default configuration is not fast enough for the amount of data we
have in production.

This commit:
- Adds a ts_vector column to the users table which we can search against
- this is recommended in the [pg_search
docs](https://github.com/Casecommons/pg_search?tab=readme-ov-file#using-tsvector-columns)
- Sneaks in a data-turbo-action="advance" attribute to the learner
search results turbo frame so we can share search result pages with
admins if we need to.

app/models/user.rb

@@ -26,7 +26,20 @@ class User < ApplicationRecord
   scope :signed_up_on, ->(date) { where(created_at: date.all_day) }
   scope :banned, -> { where(banned: true) }
 
-  pg_search_scope :search_by, against: %i[username email], using: { tsearch: { prefix: true } }
+  pg_search_scope(
+    :search_by,
+    against: %i[
+      username
+      email
+    ],
+    using: {
+      tsearch: {
+        prefix: true,
+        dictionary: 'english',
+        tsvector_column: 'search_tsvector'
+      }
+    }
+  )
 
   def progress_for(course)
     @progress ||= Hash.new { |hash, c| hash[c] = CourseProgress.new(c, self) }

app/views/admin/learners/index.html.erb

@@ -10,7 +10,7 @@
     <%= inline_svg_tag 'icons/magnifying-glass.svg', class: 'pointer-events-none col-start-1 row-start-1 ml-3 size-5 self-center text-gray-400' %>
   <% end %>
 
-  <%= turbo_frame_tag :learner_results, target: '_top' do %>
+  <%= turbo_frame_tag :learner_results, target: '_top', data: { turbo_action: 'advance' } do %>
     <div class="mt-8 flow-root">
       <% if @learners.any? %>
         <div class="-mx-4 -my-2 overflow-x-auto sm:-mx-6 max-w-6xl px-6" data-test-id="learners-list">

db/migrate/20250212143446_add_search_ts_vector_column_to_users.rb

@@ -0,0 +1,15 @@
+class AddSearchTsVectorColumnToUsers < ActiveRecord::Migration[7.1]
+  disable_ddl_transaction!
+
+  def change
+    execute <<-SQL.squish
+      ALTER TABLE users
+      ADD COLUMN search_tsvector tsvector GENERATED ALWAYS AS (
+        setweight(to_tsvector('english', coalesce(email, '')), 'A') ||
+        setweight(to_tsvector('english', coalesce(username,'')), 'B')
+      ) STORED;
+    SQL
+
+    add_index :users, :search_tsvector, using: :gin, algorithm: :concurrently
+  end
+end