activerecord_debug.rb

# enable slow query logs in Rails
ActiveRecord::Base.logger = Logger.new(STDOUT)

# Get rails to show you the sql that will run
User.where(id: 1).to_sql
=> "SELECT \"users\".* FROM \"users\" WHERE \"users\".\"id\" = 1"

# Get rails to explain the sql query for more detail
User.where(id: 1).explain

# The explain will tell you which index is being used, and what work happens in the back.
=> EXPLAIN for: SELECT "users".* FROM "users" WHERE "users"."id" = $1 [["id", 1]]
                                QUERY PLAN
--------------------------------------------------------------------------
 Index Scan using users_pkey on users  (cost=0.14..8.16 rows=1 width=962)
   Index Cond: (id = '1'::bigint)
(2 rows)