Use length not limit

db/schema.rb

@@ -331,7 +331,7 @@ ActiveRecord::Schema.define(version: 20180603190921) do
   end
   add_index "ci_builds", ["artifacts_expire_at"], name: "index_ci_builds_on_artifacts_expire_at", where: "(artifacts_file <> ''::text)", using: :btree
-  add_index "ci_builds", ["artifacts_file"], name: "index_ci_builds_on_artifacts_file", where: "(artifacts_file <> ''::text)", limit: 20, using: :btree
+  add_index "ci_builds", ["artifacts_file"], name: "index_ci_builds_on_artifacts_file", where: "(artifacts_file <> ''::text)", length: 20, using: :btree
   add_index "ci_builds", ["auto_canceled_by_id"], name: "index_ci_builds_on_auto_canceled_by_id", using: :btree
   add_index "ci_builds", ["commit_id", "stage_idx", "created_at"], name: "index_ci_builds_on_commit_id_and_stage_idx_and_created_at", using: :btree
   add_index "ci_builds", ["commit_id", "status", "type"], name: "index_ci_builds_on_commit_id_and_status_and_type", using: :btree