1 files changed, 37 insertions, 0 deletions
diff --git a/lib/gitlab/background_migration/batching_strategies/loose_index_scan_batching_strategy.rb b/lib/gitlab/background_migration/batching_strategies/loose_index_scan_batching_strategy.rb
new file mode 100644
index 00000000000..5cad9d2e3c4
--- /dev/null
+++ b/lib/gitlab/background_migration/batching_strategies/loose_index_scan_batching_strategy.rb
@@ -0,0 +1,37 @@
+# frozen_string_literal: true
+
+module Gitlab
+  module BackgroundMigration
+    module BatchingStrategies
+      # This strategy provides an efficient way to iterate over columns with non-distinct values.
+      # A common use case would be iterating over a foreign key columns, for example issues.project_id
+      class LooseIndexScanBatchingStrategy < BaseStrategy
+        include Gitlab::Database::DynamicModelHelpers
+
+        # Finds and returns the next batch in the table.
+        #
+        # table_name - The table to batch over
+        # column_name - The column to batch over
+        # batch_min_value - The minimum value which the next batch will start at
+        # batch_size - The size of the next batch
+        # job_arguments - The migration job arguments
+        # job_class - The migration job class
+        def next_batch(table_name, column_name, batch_min_value:, batch_size:, job_arguments:, job_class: nil)
+          model_class = define_batchable_model(table_name, connection: connection)
+
+          quoted_column_name = model_class.connection.quote_column_name(column_name)
+          relation = model_class.where("#{quoted_column_name} >= ?", batch_min_value)
+          next_batch_bounds = nil
+
+          relation.distinct_each_batch(of: batch_size, column: column_name) do |batch| # rubocop:disable Lint/UnreachableLoop
+            next_batch_bounds = batch.pluck(Arel.sql("MIN(#{quoted_column_name}), MAX(#{quoted_column_name})")).first
+
+            break
+          end
+
+          next_batch_bounds
+        end
+      end
+    end
+  end
+end