Denormalize commits count for merge request diffs38068-commits-count

For each MR diff an extra 'SELECT COUNT()' is executed
to get number of commits for the diff. Overall time to get counts for
all MR diffs may be quite expensive. To speed up loading of MR info,
information about number of commits is stored in a MR diff's extra column.

Closes #38068

1 files changed, 26 insertions, 0 deletions

diff --git a/lib/gitlab/background_migration/add_merge_request_diff_commits_count.rb b/lib/gitlab/background_migration/add_merge_request_diff_commits_count.rb
new file mode 100644
index 00000000000..7bffffec94d
--- /dev/null
+++ b/lib/gitlab/background_migration/add_merge_request_diff_commits_count.rb
@@ -0,0 +1,26 @@
+# frozen_string_literal: true
+# rubocop:disable Style/Documentation
+# rubocop:disable Metrics/LineLength
+
+module Gitlab
+  module BackgroundMigration
+    class AddMergeRequestDiffCommitsCount
+      class MergeRequestDiff < ActiveRecord::Base
+        self.table_name = 'merge_request_diffs'
+      end
+
+      def perform(start_id, stop_id)
+        Rails.logger.info("Setting commits_count for merge request diffs: #{start_id} - #{stop_id}")
+
+        update = '
+          commits_count = (
+            SELECT count(*)
+            FROM merge_request_diff_commits
+            WHERE merge_request_diffs.id = merge_request_diff_commits.merge_request_diff_id
+          )'.squish
+
+        MergeRequestDiff.where(id: start_id..stop_id).update_all(update)
+      end
+    end
+  end
+end