Add unique index on services project_id and type

This replaces the previous non-unique index on the same columns.

changelogs/unreleased/207385-services-unique-constraint-on-type.yml

+---
+title: Add unique index on services project_id and type
+merge_request: 52563
+author:
+type: changed

db/migrate/20210126091713_add_unique_index_services_project_id_and_type.rb

+# frozen_string_literal: true
+
+class AddUniqueIndexServicesProjectIdAndType < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  INDEX_NAME = 'index_services_on_project_id_and_type_unique'
+
+  def up
+    add_concurrent_index :services, [:project_id, :type], name: INDEX_NAME, unique: true
+  end
+
+  def down
+    remove_concurrent_index_by_name :services, name: INDEX_NAME
+  end
+end

db/migrate/20210126092102_remove_index_services_project_id_and_type.rb

+# frozen_string_literal: true
+
+class RemoveIndexServicesProjectIdAndType < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  INDEX_NAME = 'index_services_on_project_id_and_type'
+
+  # Replaced by the index added in 20210126091713_add_unique_index_services_project_id_and_type.rb
+  def up
+    remove_concurrent_index_by_name :services, name: INDEX_NAME
+  end
+
+  def down
+    add_concurrent_index :services, [:project_id, :type], name: INDEX_NAME
+  end
+end

db/schema_migrations/20210126091713

+3906739d07514e6e59f79a4a81d28859a2481614a299c95ec1b1d9825a07ec64
\ No newline at end of file

db/schema_migrations/20210126092102

+124c5ae1a1ccade5dec01f72b726e03febc8f56411d7d8990f976bb2a9516037
\ No newline at end of file

db/structure.sql

@@ -23003,7 +23003,7 @@ CREATE INDEX index_service_desk_enabled_projects_on_id_creator_id_created_at ON
 
 CREATE INDEX index_services_on_inherit_from_id ON services USING btree (inherit_from_id);
 
-CREATE INDEX index_services_on_project_id_and_type ON services USING btree (project_id, type);
+CREATE UNIQUE INDEX index_services_on_project_id_and_type_unique ON services USING btree (project_id, type);
 
 CREATE INDEX index_services_on_template ON services USING btree (template);