Enable DISTINCT optimization for ObjectHierarchy globally

This uses the feature flag and work in
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/56509 to ensure
the query planner chooses the nested join instead of the more expensive
hashed join. When the `use_distinct_for_all_object_hierarchy` feature
flag is used globally, it will apply to all queries, not just the ones
that have manually enabled them via the `options` hash while creating
`ObjectHierarchy`.

This change works around a PostgreSQL query planner bug that is detailed
in https://gitlab.com/gitlab-com/gl-infra/production/-/issues/3894.

Related issues:

1. https://gitlab.com/gitlab-com/gl-infra/production/-/issues/4024
2. https://gitlab.com/groups/gitlab-org/-/epics/5617

changelogs/unreleased/sh-global-use-distinct-object-hierarchy.yml

+---
+title: Enable DISTINCT optimization for ObjectHierarchy globally
+merge_request: 57047
+author:
+type: changed

config/feature_flags/development/use_distinct_for_all_object_hierarchy.yml

+---
+name: use_distinct_for_all_object_hierarchy
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/57047
+rollout_issue_url: 
+milestone: '13.11'
+type: development
+group: group::database
+default_enabled: false

lib/gitlab/object_hierarchy.rb

@@ -96,7 +96,7 @@ module Gitlab
         base_cte = base_and_descendants_cte(with_depth: true).apply_to(model.all).distinct
 
         if with_depth
-          read_only(model.from(Arel::Nodes::As.new(recursive_query.arel, objects_table)).order(depth: :asc))
+          read_only(model.from(Arel::Nodes::As.new(base_cte.arel, objects_table)).order(depth: :asc))
         else
           read_only(remove_depth_and_maintain_order(base_cte, hierarchy_order: :asc))
         end
@@ -161,7 +161,12 @@ module Gitlab
 
     # Use distinct on the Namespace queries to avoid bad planner behavior in PG11.
     def use_distinct?
-      (model <= Namespace) && options[:use_distinct]
+      return unless model <= Namespace
+      # Global use_distinct_for_all_object_hierarchy takes precedence over use_distinct_in_object_hierarchy
+      return true if Feature.enabled?(:use_distinct_for_all_object_hierarchy)
+      return options[:use_distinct] if options.key?(:use_distinct)
+
+      false
     end
 
     # Remove the extra `depth` field using an INNER JOIN to avoid breaking UNION queries

spec/lib/gitlab/object_hierarchy_spec.rb

@@ -187,6 +187,21 @@ RSpec.describe Gitlab::ObjectHierarchy do
   context 'when the use_distinct_in_object_hierarchy feature flag is enabled' do
     before do
       stub_feature_flags(use_distinct_in_object_hierarchy: true)
+      stub_feature_flags(use_distinct_for_all_object_hierarchy: false)
+    end
+
+    it_behaves_like 'Gitlab::ObjectHierarchy test cases'
+
+    it 'calls DISTINCT' do
+      expect(parent.self_and_descendants.to_sql).to include("DISTINCT")
+      expect(child2.self_and_ancestors.to_sql).to include("DISTINCT")
+    end
+  end
+
+  context 'when the use_distinct_for_all_object_hierarchy feature flag is enabled' do
+    before do
+      stub_feature_flags(use_distinct_in_object_hierarchy: false)
+      stub_feature_flags(use_distinct_for_all_object_hierarchy: true)
     end
 
     it_behaves_like 'Gitlab::ObjectHierarchy test cases'
@@ -200,6 +215,7 @@ RSpec.describe Gitlab::ObjectHierarchy do
   context 'when the use_distinct_in_object_hierarchy feature flag is disabled' do
     before do
       stub_feature_flags(use_distinct_in_object_hierarchy: false)
+      stub_feature_flags(use_distinct_for_all_object_hierarchy: false)
     end
 
     it_behaves_like 'Gitlab::ObjectHierarchy test cases'