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

Reduce UPDATEs for background column type changes

Closes #42158

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

(cherry picked from commit 792e9ed7fa46d236c01fb14c8ad7f9b4ea4dee59)

0054d383 Reduce UPDATEs for background column type changes

3 files changed, 6 insertions, 3 deletions

diff --git a/lib/gitlab/background_migration/copy_column.rb b/lib/gitlab/background_migration/copy_column.rb
index a2cb215c230..ef70f37d5eb 100644
--- a/lib/gitlab/background_migration/copy_column.rb
+++ b/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
 
diff --git a/lib/gitlab/database/migration_helpers.rb b/lib/gitlab/database/migration_helpers.rb
index 7b35c24d153..98de29f12a3 100644
--- a/lib/gitlab/database/migration_helpers.rb
+++ b/lib/gitlab/database/migration_helpers.rb
@@ -524,8 +524,9 @@ 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
 
diff --git a/spec/lib/gitlab/database/migration_helpers_spec.rb b/spec/lib/gitlab/database/migration_helpers_spec.rb
index 43761c2fe0c..1de3a14b809 100644
--- a/spec/lib/gitlab/database/migration_helpers_spec.rb
+++ b/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