Separate add_concurrent_index migration from background migration

db/migrate/20180428200509_add_index_to_ci_builds_artifacts_file.rb

+class AddIndexToCiBuildsArtifactsFile < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def up
+    # We add an temporary index to `ci_builds.artifacts_file` column to avoid statements timeout in legacy artifacts migrations
+    # This index is to be removed after we have cleaned up background migrations
+    # https://gitlab.com/gitlab-org/gitlab-ce/issues/46866
+    add_concurrent_index :ci_builds, :artifacts_file, where: "artifacts_file <> ''"
+  end
+
+  def down
+    remove_concurrent_index :ci_builds, :artifacts_file, where: "artifacts_file <> ''"
+  end
+end

db/post_migrate/20180427161409_migrate_legacy_artifacts_to_job_artifacts.rb → db/post_migrate/20180430161409_migrate_legacy_artifacts_to_job_artifacts.rb

@@ -30,13 +30,6 @@ class MigrateLegacyArtifactsToJobArtifacts < ActiveRecord::Migration
   end
 
   def up
-    # We add an temporary index to `ci_builds.artifacts_file` column to avoid statements timeout
-    # This index is to be removed after we have cleaned up background migrations
-    # https://gitlab.com/gitlab-org/gitlab-ce/issues/46866
-    unless index_exists?(:ci_builds, :artifacts_file)
-      add_concurrent_index :ci_builds, :artifacts_file, where: "artifacts_file <> ''"
-    end
-
     MigrateLegacyArtifactsToJobArtifacts::Build.legacy_artifacts.without_new_artifacts.tap do |relation|
       queue_background_migration_jobs_by_range_at_intervals(relation,
                                                             MIGRATION,
@@ -46,8 +39,6 @@ class MigrateLegacyArtifactsToJobArtifacts < ActiveRecord::Migration
   end
 
   def down
-    if index_exists?(:ci_builds, :artifacts_file)
-      remove_concurrent_index :ci_builds, :artifacts_file, where: "artifacts_file <> ''"
-    end
+    # no-op
   end
 end