Partial index for namespaces.type

What does this MR do?

This removes null values from the index on namespaces (type) (with a partial condition WHERE type is NOT NULL).

For the reasoning and more background, please refer to https://gitlab.com/gitlab-org/gitlab-ee/issues/31540#note_216251003.

This replaces the CE MR: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/32876

Screenshots

abrandl-gl:gitlab/ (ab/partial-index-for-namespaces-type✗) $ spring rake db:migrate                                                                                                                                                                                            [14:04:35]
Running via Spring preloader in process 32334
== 20190910114843 ExcludeNullsFromIndexOnNamespacesType: migrating ============
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:namespaces, :type, {:where=>"type is not null", :name=>"index_namespaces_on_type_partial", :algorithm=>:concurrently})
   -> 0.0064s
-- execute("SET statement_timeout TO 0")
   -> 0.0003s
-- add_index(:namespaces, :type, {:where=>"type is not null", :name=>"index_namespaces_on_type_partial", :algorithm=>:concurrently})
   -> 0.0179s
-- execute("RESET ALL")
   -> 0.0004s
-- transaction_open?()
   -> 0.0000s
-- select_one("SELECT current_setting('server_version_num') AS v")
   -> 0.0004s
-- indexes(:namespaces)
   -> 0.0059s
-- execute("SET statement_timeout TO 0")
   -> 0.0002s
-- remove_index(:namespaces, {:algorithm=>:concurrently, :name=>"index_namespaces_on_type"})
   -> 0.0020s
-- execute("RESET ALL")
   -> 0.0002s
== 20190910114843 ExcludeNullsFromIndexOnNamespacesType: migrated (0.0344s) ===

abrandl-gl:gitlab/ (ab/partial-index-for-namespaces-type) $ spring rake db:migrate:down VERSION=20190910114843                                                                                                                                                                 [14:04:44]
Running via Spring preloader in process 32385
== 20190910114843 ExcludeNullsFromIndexOnNamespacesType: reverting ============
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:namespaces, :type, {:name=>"index_namespaces_on_type", :algorithm=>:concurrently})
   -> 0.0084s
-- execute("SET statement_timeout TO 0")
   -> 0.0003s
-- add_index(:namespaces, :type, {:name=>"index_namespaces_on_type", :algorithm=>:concurrently})
   -> 0.0149s
-- execute("RESET ALL")
   -> 0.0004s
-- transaction_open?()
   -> 0.0000s
-- select_one("SELECT current_setting('server_version_num') AS v")
   -> 0.0004s
-- indexes(:namespaces)
   -> 0.0053s
-- execute("SET statement_timeout TO 0")
   -> 0.0002s
-- remove_index(:namespaces, {:algorithm=>:concurrently, :name=>"index_namespaces_on_type_partial"})
   -> 0.0018s
-- execute("RESET ALL")
   -> 0.0002s
== 20190910114843 ExcludeNullsFromIndexOnNamespacesType: reverted (0.0324s) ===

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.