Add a temporary specialized index on project_namespace_id

Changelog: added

db/post_migrate/20220323130000_add_temp_index_on_null_project_namespace_ids.rb

+# frozen_string_literal: true
+
+class AddTempIndexOnNullProjectNamespaceIds < Gitlab::Database::Migration[1.0]
+  TMP_INDEX_FOR_NULL_PROJECT_NAMESPACE_ID = 'tmp_index_for_null_project_namespace_id'
+
+  disable_ddl_transaction!
+
+  def up
+    add_concurrent_index :projects, :id, name: TMP_INDEX_FOR_NULL_PROJECT_NAMESPACE_ID, where: 'project_namespace_id IS NULL'
+  end
+
+  def down
+    remove_concurrent_index_by_name :projects, name: TMP_INDEX_FOR_NULL_PROJECT_NAMESPACE_ID
+  end
+end

db/schema_migrations/20220323130000

+6152ff1a3647e1ac26e8f462215a0dec7d96c5552373f4ad99424c7c74e80577
\ No newline at end of file

db/structure.sql

@@ -29634,6 +29634,8 @@ CREATE INDEX tmp_index_for_namespace_id_migration_on_group_members ON members US
 
 CREATE INDEX tmp_index_for_namespace_id_migration_on_routes ON routes USING btree (id) WHERE ((namespace_id IS NULL) AND ((source_type)::text = 'Namespace'::text));
 
+CREATE INDEX tmp_index_for_null_project_namespace_id ON projects USING btree (id) WHERE (project_namespace_id IS NULL);
+
 CREATE INDEX tmp_index_issues_on_issue_type_and_id ON issues USING btree (issue_type, id);
 
 CREATE INDEX tmp_index_members_on_state ON members USING btree (state) WHERE (state = 2);