Add unique index for work item type names with no namespace

The existing index for unique names on the work_item_types
table won't account for records that have a null namespace_id.
This change will create a unique index only for those work item
types that have a null namespace_id

Changelog: other

db/migrate/20220408001450_add_work_item_type_name_unique_index_null_namespaces.rb

+# frozen_string_literal: true
+
+class AddWorkItemTypeNameUniqueIndexNullNamespaces < Gitlab::Database::Migration[1.0]
+  INDEX_NAME = :idx_work_item_types_on_namespace_id_and_name_null_namespace
+
+  disable_ddl_transaction!
+
+  def up
+    add_concurrent_index :work_item_types,
+                         'TRIM(BOTH FROM LOWER(name)), (namespace_id IS NULL)',
+                         unique: true,
+                         name: INDEX_NAME,
+                         where: 'namespace_id IS NULL'
+  end
+
+  def down
+    remove_concurrent_index_by_name :work_item_types, INDEX_NAME
+  end
+end

db/schema_migrations/20220408001450

+42d7864cdcec589b7de8bc3a872e2743496d81788a60ddcd213f049fda12474e
\ No newline at end of file

db/structure.sql

@@ -26636,6 +26636,8 @@ CREATE UNIQUE INDEX idx_vulnerability_issue_links_on_vulnerability_id_and_issue_
 
 CREATE UNIQUE INDEX idx_vulnerability_issue_links_on_vulnerability_id_and_link_type ON vulnerability_issue_links USING btree (vulnerability_id, link_type) WHERE (link_type = 2);
 
+CREATE UNIQUE INDEX idx_work_item_types_on_namespace_id_and_name_null_namespace ON work_item_types USING btree (btrim(lower(name)), ((namespace_id IS NULL))) WHERE (namespace_id IS NULL);
+
 CREATE INDEX index_abuse_reports_on_user_id ON abuse_reports USING btree (user_id);
 
 CREATE INDEX index_agent_activity_events_on_agent_id_and_recorded_at_and_id ON agent_activity_events USING btree (agent_id, recorded_at, id);