Add length back for indexing

db/migrate/20180428200509_add_index_to_ci_builds_artifacts_file.rb

@@ -11,7 +11,7 @@ class AddIndexToCiBuildsArtifactsFile < ActiveRecord::Migration
     # 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 <> ''"
+    add_concurrent_index :ci_builds, :artifacts_file, where: "artifacts_file <> ''", length: 20
   end
 
   def down