Denormalize commits count for merge request diffs

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, 29 insertions, 0 deletions

diff --git a/db/migrate/20180105212544_add_commits_count_to_merge_request_diff.rb b/db/migrate/20180105212544_add_commits_count_to_merge_request_diff.rb
new file mode 100644
index 00000000000..f942b4c062e
--- /dev/null
+++ b/db/migrate/20180105212544_add_commits_count_to_merge_request_diff.rb
@@ -0,0 +1,29 @@
+class AddCommitsCountToMergeRequestDiff < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  MIGRATION = 'AddMergeRequestDiffCommitsCount'.freeze
+  BATCH_SIZE = 5000
+  DELAY_INTERVAL = 5.minutes.to_i
+
+  class MergeRequestDiff < ActiveRecord::Base
+    self.table_name = 'merge_request_diffs'
+
+    include ::EachBatch
+  end
+
+  disable_ddl_transaction!
+
+  def up
+    add_column :merge_request_diffs, :commits_count, :integer
+
+    say 'Populating the MergeRequestDiff `commits_count`'
+
+    queue_background_migration_jobs_by_range_at_intervals(MergeRequestDiff, MIGRATION, DELAY_INTERVAL, batch_size: BATCH_SIZE)
+  end
+
+  def down
+    remove_column :merge_request_diffs, :commits_count
+  end
+end