Replace namespaces unique index on name and parent_id

What does this MR do and why?

This MR replaces existing unique index on namespaces table on name, parent_id columns with a unique index on name, parent_id, type columns.

Given that we currently only have nil and Group as namespace types and namespaces with type nil(which is a user namespace) cannot be a child of a Group because user namespaces cannot have a parent, new index will still keep the current functionality intact.

Why do we need the new index then?

There are 2 factors that force us to slightly change the unique index:

1. We are working towards consolidating Projects and Namespaces (see &6473). For that we will introduce ProjectNamespace model, that will have a corresponding record in namespaces table for each Project. ProjectNamespace will have type=Project.
2. Currently we do not allow 2 child groups to have the same name within a group, but we do allow a project to have the same name as any of the child groups if using a different path. Here is a screenshot to better illustrate what I mean:

Even though on .com there are no such occurrences where a project in a group would have the same name as any 'child group' in that group, it is not impossible that there will be such cases on on-premise installs.

So slightly changing the index keeps both the current functionality and also allows us to add ProjectNamespace records to namespaces table even if a project would have the same name as some child group within its parent group. Which would not be possible with the current unique index just on name, parent_id columns

re #341814 (closed)

Database

UP

== 20210922172056 AddUniqueNamespacesIndexOnNameParentIdAndType: migrating ====
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:namespaces, [:name, :parent_id, :type], {:unique=>true, :name=>"index_namespaces_name_parent_id_type", :algorithm=>:concurrently})
   -> 0.0152s
-- execute("SET statement_timeout TO 0")
   -> 0.0007s
-- add_index(:namespaces, [:name, :parent_id, :type], {:unique=>true, :name=>"index_namespaces_name_parent_id_type", :algorithm=>:concurrently})
   -> 0.0065s
-- execute("RESET statement_timeout")
   -> 0.0010s
== 20210922172056 AddUniqueNamespacesIndexOnNameParentIdAndType: migrated (0.0280s)

== 20210922172156 DropUniqueNamespacesIndexOnNameAndParentId: migrating =======
-- transaction_open?()
   -> 0.0000s
-- indexes(:namespaces)
   -> 0.0111s
-- remove_index(:namespaces, {:algorithm=>:concurrently, :name=>"index_namespaces_on_name_and_parent_id"})
   -> 0.0031s
== 20210922172156 DropUniqueNamespacesIndexOnNameAndParentId: migrated (0.0159s)

DOWN

== 20210922172156 DropUniqueNamespacesIndexOnNameAndParentId: reverting =======
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:namespaces, [:name, :parent_id], {:unique=>true, :name=>"index_namespaces_on_name_and_parent_id", :algorithm=>:concurrently})
   -> 0.0161s
-- add_index(:namespaces, [:name, :parent_id], {:unique=>true, :name=>"index_namespaces_on_name_and_parent_id", :algorithm=>:concurrently})
   -> 0.0092s
== 20210922172156 DropUniqueNamespacesIndexOnNameAndParentId: reverted (0.0276s)

== 20210922172056 AddUniqueNamespacesIndexOnNameParentIdAndType: reverting ====
-- transaction_open?()
   -> 0.0000s
-- indexes(:namespaces)
   -> 0.0151s
-- remove_index(:namespaces, {:algorithm=>:concurrently, :name=>"index_namespaces_name_parent_id_type"})
   -> 0.0041s
== 20210922172056 AddUniqueNamespacesIndexOnNameParentIdAndType: reverted (0.0214s)

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