find_each.rb

# Selecting all users
User.all.each do |user|
  notify(user)
end

# Generates this query:
# SELECT * FROM users;

# Selecting all users with `find_each` will query by batches of size
# determined by the :batch_size option (defaults to 1000)
User.all.find_each(batch_size: 5000) do |user|
  notify(user)
end

# Generates the queries
# SELECT * FROM users ORDER BY users.id ASC LIMIT 5000;
# SELECT * FROM users WHERE users.id > 5000 ORDER BY users.id ASC LIMIT 5000;
# SELECT * FROM users WHERE users.id > 10000 ORDER BY users.id ASC LIMIT 5000;
# ....