Merge branch 'background-migration-fix' into 'master'

Reduce UPDATEs for background column type changes

Closes #42158

See merge request gitlab-org/gitlab-ce!16551

lib/gitlab/background_migration/copy_column.rb

@@ -28,6 +28,8 @@ module Gitlab
         UPDATE #{quoted_table}
         SET #{quoted_copy_to} = #{quoted_copy_from}
         WHERE id BETWEEN #{start_id} AND #{end_id}
+        AND #{quoted_copy_from} IS NOT NULL
+        AND #{quoted_copy_to} IS NULL
         SQL
       end

lib/gitlab/database/migration_helpers.rb

@@ -525,8 +525,9 @@ module Gitlab
         install_rename_triggers(table, column, temp_column)
         # Schedule the jobs that will copy the data from the old column to the
-        # new one.
-        relation.each_batch(of: batch_size) do |batch, index|
+        # new one. Rows with NULL values in our source column are skipped since
+        # the target column is already NULL at this point.
+        relation.where.not(column => nil).each_batch(of: batch_size) do |batch, index|
           start_id, end_id = batch.pluck('MIN(id), MAX(id)').first
           max_index = index

spec/lib/gitlab/database/migration_helpers_spec.rb

@@ -1038,7 +1038,7 @@ describe Gitlab::Database::MigrationHelpers do
   end
   describe '#change_column_type_using_background_migration' do
-    let!(:issue) { create(:issue) }
+    let!(:issue) { create(:issue, :closed, closed_at: Time.zone.now) }
     let(:issue_model) do
       Class.new(ActiveRecord::Base) do