in_memory.rb

# Suppose we want to select from these n emails those that
# are not already in the `users` table.
emails = [
  "[email protected]",
  "[email protected]",
  "[email protected]",
  ...
  "[email protected]"
]

new_emails = []

emails.each do |email|
  unless User.where(email: email).exists?
    new_emails << email
  end
end
# The above will run n queries, one for each email
# SELECT 1 FROM users WHERE email` = '[email protected]' LIMIT 1;
# SELECT 1 FROM users WHERE email` = '[email protected]' LIMIT 1;
# SELECT 1 FROM users WHERE email` = '[email protected]' LIMIT 1;
# ...
# SELECT 1 FROM users WHERE email` = '[email protected]' LIMIT 1;

# Instead, let's grab all the emails from the database first in one query,
# then filter in memory and store in a Set (since we want to only
# check for inclusion).
existing_emails = Set.new(User.pluck(:email))
# SELECT users.email from users;

new_emails = []

emails.each do |email|
  unless existing_emails.include?(email)
    new_emails << email
  end
end