PERF: PG queries for the `UserEmail#email` column was not using the index.

app/controllers/finish_installation_controller.rb

@@ -15,7 +15,7 @@ class FinishInstallationController < ApplicationController
       email = params[:email].strip
       raise Discourse::InvalidParameters.new unless @allowed_emails.include?(email)
 
-      return redirect_confirm(email) if UserEmail.exists?(email: email)
+      return redirect_confirm(email) if UserEmail.where("lower(email) = ?", email).exists?
 
       @user.email = email
       @user.username = params[:username]

app/models/user.rb

@@ -134,7 +134,9 @@ class User < ActiveRecord::Base
   # set to true to optimize creation and save for imports
   attr_accessor :import_mode
 
-  scope :with_email, ->(email) { joins(:user_emails).where(user_emails: { email: email }) }
+  scope :with_email, ->(email) do
+    joins(:user_emails).where("lower(user_emails.email) = ?", email)
+  end
 
   scope :human_users, -> { where('users.id > 0') }
 

lib/admin_user_index_query.rb

@@ -104,7 +104,7 @@ class AdminUserIndexQuery
   def filter_by_user_with_bypass(filter)
     if filter =~ /.+@.+/
       # probably an email so try the bypass
-      user_id = UserEmail.where(email: filter.downcase).pluck(:user_id).first
+      user_id = UserEmail.where("lower(email) = ?", filter.downcase).pluck(:user_id).first
       if user_id
         return @query.where('users.id = ?', user_id)
       end