Add a partial index to find rows with NULL store

So a background migration can easily update those rows.

db/post_migrate/20200804035230_add_partial_index_on_id_to_merge_request_diffs.rb

+# frozen_string_literal: true
+
+class AddPartialIndexOnIdToMergeRequestDiffs < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+  INDEX_NAME = 'index_merge_request_diffs_external_diff_store_is_null'
+
+  disable_ddl_transaction!
+
+  def up
+    add_concurrent_index :merge_request_diffs, :id, where: 'external_diff_store IS NULL', name: INDEX_NAME
+  end
+
+  def down
+    remove_concurrent_index_by_name :merge_request_diffs, INDEX_NAME
+  end
+end

db/schema_migrations/20200804035230

+f1e2f11710ca7175b7a77b0f52bd204403dfcfd0b6b33cd00e3fa0d020be1287
\ No newline at end of file

db/structure.sql

@@ -19849,6 +19849,8 @@ CREATE INDEX index_merge_request_diff_commits_on_sha ON public.merge_request_dif
 
 CREATE UNIQUE INDEX index_merge_request_diff_files_on_mr_diff_id_and_order ON public.merge_request_diff_files USING btree (merge_request_diff_id, relative_order);
 
+CREATE INDEX index_merge_request_diffs_external_diff_store_is_null ON public.merge_request_diffs USING btree (id) WHERE (external_diff_store IS NULL);
+
 CREATE INDEX index_merge_request_diffs_on_external_diff_store ON public.merge_request_diffs USING btree (external_diff_store);
 
 CREATE INDEX index_merge_request_diffs_on_merge_request_id_and_id ON public.merge_request_diffs USING btree (merge_request_id, id);