Separate adding and removing index from stages migration

db/post_migrate/20180212101828_add_tmp_partial_null_index_to_builds.rb

+class AddTmpPartialNullIndexToBuilds < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  disable_ddl_transaction!
+
+  def up
+    add_concurrent_index(:ci_builds, :stage_id, where: 'stage_id IS NULL',
+                                                name: 'tmp_stage_id_partial_null_index')
+  end
+
+  def down
+    remove_concurrent_index_by_name(:ci_builds, 'tmp_stage_id_partial_null_index')
+  end
+end

db/post_migrate/20180212101928_schedule_build_stage_migration.rb

@@ -15,17 +15,12 @@ class ScheduleBuildStageMigration < ActiveRecord::Migration
   def up
     disable_statement_timeout
 
-    add_concurrent_index(:ci_builds, :stage_id, where: 'stage_id IS NULL',
-                                                name: 'tmp_stage_id_partial_null_index')
-
     Build.where('stage_id IS NULL').tap do |relation|
       queue_background_migration_jobs_by_range_at_intervals(relation,
                                                             MIGRATION,
                                                             5.minutes,
                                                             batch_size: BATCH_SIZE)
     end
-
-    remove_concurrent_index_by_name(:ci_builds, 'tmp_stage_id_partial_null_index')
   end
 
   def down

db/post_migrate/20180212102028_remove_tmp_partial_null_index_from_builds.rb

+class RemoveTmpPartialNullIndexFromBuilds < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  disable_ddl_transaction!
+
+  def up
+    remove_concurrent_index_by_name(:ci_builds, 'tmp_stage_id_partial_null_index')
+  end
+
+  def down
+    add_concurrent_index(:ci_builds, :stage_id, where: 'stage_id IS NULL',
+                                                name: 'tmp_stage_id_partial_null_index')
+  end
+end

db/schema.rb

@@ -11,7 +11,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 20180212101928) do
+ActiveRecord::Schema.define(version: 20180212102028) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"