How do I check ActiveRecord memory usage?

If you insert middleware like this, it will come off.

class QueryCacheProf
  def initialize (app)
    @app=app
  end

  def call(env)
    @ app.call(env).tap do
      GC.start
      memsize_of_all_before_query_cache_cleared=ObjectSpace.memsize_of_all
      ActiveRecord::Base.connection.clear_query_cache
      GC.start
      memsize_of_query_cache = memsize_of_all_before_query_cache_cleared-ObjectSpace.memsize_of_all
      Rails.logger.info "memsize of query cache: #{memsize_of_query_cache}"
    end
  end
end

Rails.application.config.middleware.insert_after ActiveRecord::QueryCache, QueryCacheProf

If you run bundle execrake middleware, you will see the line use ActiveRecord::QueryCache.

In the Rack middleware that ActiveRecord plugs into, this middleware caches the results of the same query being requested.

Specifically, the following parts are inserted.
https://github.com/rails/rails/blob/0c7916952e0f6110e10a2968cbdab5326f571924/activerecord/lib/active_record/railtie.rb#L19,L20

Specifically, if you read the QueryCache middleware implementation, you can call connection.enable_query_cache! and then process the request.
https://github.com/rails/rails/blob/0c7916952e0f6110e10a2968cbdab5326f571924/activerecord/lib/active_record/query_cache.rb#L34

After calling enable_query_cache!, @query_cache_enabled is set to
https://github.com/rails/rails/blob/0c7916952e0f6110e10a2968cbdab5326f571924/activerecord/lib/active_record/connection_adapters/abstract/query_cache.rb#L39

If @query_cache_enabled is configured, the results of the select statement are cached in cache_sql.
https://github.com/rails/rails/blob/0c7916952e0f6110e10a2968cbdab5326f571924/activerecord/lib/active_record/connection_adapters/abstract/query_cache.rb#L65,L68

If you read cache_sql, you will see that @query_cache has cached the results.
If you measure the size of the cache here, you will see how much memory you are consuming.

"If you google with the keyword ""ruby memory profiler"", you will get a lot of information."

• Ten tools (such as profiler) to follow Ruby processes - sonots:blog
• Line Memory Profiler in Ruby - Qiita
• Memory Usage Survey in Ruby - Qiita
• SamSaffron/memory_profiler
• ruby-prof/ruby-prof

I've never used it before, so I can't show you how to do it, but if you look at the sources above, you may get answers to the questions.