gitlab-org--gitlab-foss/doc/development/iterating_tables_in_batches.md

# Iterating Tables In Batches

Rails provides a method called `in_batches` that can be used to iterate over
rows in batches. For example:

```ruby
User.in_batches(of: 10) do |relation|
  relation.update_all(updated_at: Time.now)
end
```

Unfortunately this method is implemented in a way that is not very efficient,
both query and memory usage wise.

To work around this you can include the `EachBatch` module into your models,
then use the `each_batch` class method. For example:

```ruby
class User < ActiveRecord::Base
  include EachBatch
end

User.each_batch(of: 10) do |relation|
  relation.update_all(updated_at: Time.now)
end
```

This will end up producing queries such as:

```plaintext
User Load (0.7ms)  SELECT  "users"."id" FROM "users" WHERE ("users"."id" >= 41654)  ORDER BY "users"."id" ASC LIMIT 1 OFFSET 1000
  (0.7ms)  SELECT COUNT(*) FROM "users" WHERE ("users"."id" >= 41654) AND ("users"."id" < 42687)
```

The API of this method is similar to `in_batches`, though it doesn't support
all of the arguments that `in_batches` supports. You should always use
`each_batch` _unless_ you have a specific need for `in_batches`.