Add each_batch_range to developer documentation

doc/development/migration_style_guide.md

@@ -943,6 +943,41 @@ derived from the class name or namespace.
 
 Be aware of the limitations [when using models in migrations](#using-models-in-migrations-discouraged).
 
+### Modifying existing data
+
+In most circumstances, prefer migrating data in **batches** when modifying data in the database.
+
+We introduced a new helper [each_batch_range](https://gitlab.com/gitlab-org/gitlab/-/blob/cd3e0a5cddcb464cb9b8c6e3275839cf57dfa6e2/lib/gitlab/database/dynamic_model_helpers.rb#L28-32) which facilitates the process of iterating over a collection in a performant way. The default size of the batch is defined in the `BATCH_SIZE` constant.
+
+See the following example to get an idea.
+
+**Purging data in batch:**
+
+```ruby
+include ::Gitlab::Database::DynamicModelHelpers
+
+disable_ddl_transaction!
+
+def up
+  each_batch_range('ci_pending_builds', scope: ->(table) { table.ref_protected }, of: BATCH_SIZE) do |min, max|
+    execute <<~SQL
+      DELETE FROM ci_pending_builds
+        USING ci_builds
+        WHERE ci_builds.id = ci_pending_builds.build_id
+          AND ci_builds.status != 'pending'
+          AND ci_builds.type = 'Ci::Build'
+          AND ci_pending_builds.id BETWEEN #{min} AND #{max}
+    SQL
+  end
+end
+```
+
+- The first argument is the table being modified: `'ci_pending_builds'`.
+- The second argument calls a lambda which fetches the relevant dataset selected (the default is set to `.all`): `scope: ->(table) { table.ref_protected }`.
+- The third argument is the batch size (the default is set in the `BATCH_SIZE` constant): `of: BATCH_SIZE`.
+
+Here is an [example MR](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/62195) illustrating how to use our new helper.
+
 ### Renaming reserved paths
 
 When a new route for projects is introduced, it could conflict with any

lib/gitlab/database/dynamic_model_helpers.rb

@@ -3,6 +3,8 @@
 module Gitlab
   module Database
     module DynamicModelHelpers
+      BATCH_SIZE = 1_000
+
       def define_batchable_model(table_name)
         Class.new(ActiveRecord::Base) do
           include EachBatch
@@ -12,7 +14,7 @@ module Gitlab
         end
       end
 
-      def each_batch(table_name, scope: ->(table) { table.all }, of: 1000)
+      def each_batch(table_name, scope: ->(table) { table.all }, of: BATCH_SIZE)
         if transaction_open?
           raise <<~MSG.squish
             each_batch should not run inside a transaction, you can disable
@@ -25,7 +27,7 @@ module Gitlab
           .each_batch(of: of) { |batch| yield batch }
       end
 
-      def each_batch_range(table_name, scope: ->(table) { table.all }, of: 1000)
+      def each_batch_range(table_name, scope: ->(table) { table.all }, of: BATCH_SIZE)
         each_batch(table_name, scope: scope, of: of) do |batch|
           yield batch.pluck('MIN(id), MAX(id)').first
         end