Partial index for namespaces.type

Relates to https://gitlab.com/gitlab-org/gitlab-ce/issues/66956

changelogs/unreleased/ab-partial-index-for-namespaces-type.yml

+---
+title: Partial index for namespaces.type
+merge_request: 32876
+author:
+type: performance

db/migrate/20190910114843_exclude_nulls_from_index_on_namespaces_type.rb

+# frozen_string_literal: true
+
+class ExcludeNullsFromIndexOnNamespacesType < ActiveRecord::Migration[5.1]
+  include Gitlab::Database::MigrationHelpers
+
+  # Set this constant to true if this migration requires downtime.
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def up
+    add_concurrent_index(:namespaces, :type, where: 'type is not null', name: 'index_namespaces_on_type_partial')
+    remove_concurrent_index_by_name(:namespaces, 'index_namespaces_on_type')
+  end
+
+  def down
+    add_concurrent_index(:namespaces, :type, name: 'index_namespaces_on_type')
+    remove_concurrent_index_by_name(:namespaces, 'index_namespaces_on_type_partial')
+  end
+end

db/schema.rb

@@ -2291,7 +2291,7 @@ ActiveRecord::Schema.define(version: 2019_09_12_061145) do
     t.index ["runners_token_encrypted"], name: "index_namespaces_on_runners_token_encrypted", unique: true
     t.index ["shared_runners_minutes_limit", "extra_shared_runners_minutes_limit"], name: "index_namespaces_on_shared_and_extra_runners_minutes_limit"
     t.index ["trial_ends_on"], name: "index_namespaces_on_trial_ends_on", where: "(trial_ends_on IS NOT NULL)"
-    t.index ["type"], name: "index_namespaces_on_type"
+    t.index ["type"], name: "index_namespaces_on_type_partial", where: "(type IS NOT NULL)"
   end
 
   create_table "note_diff_files", id: :serial, force: :cascade do |t|