Active Record Query Interface

Ruby on Rails Guides: Active Record Query Interface

But this approach becomes increasingly impractical as the table size increases, since User.all.each instructs Active Record to fetch the entire table in a single pass, build a model object per row, and then keep the entire array of model objects in memory. Indeed, if we have a large number of records, the entire collection may exceed the amount of memory available.

Full article: http://guides.rubyonrails.org/active_record_querying.html