Fix maven package query performance

This change fixes the maven package finder query performance by using a
CTE fence.

app/finders/packages/maven/package_finder.rb

@@ -33,7 +33,7 @@ module Packages
       end
 
       def packages_with_path
-        matching_packages = base.only_maven_packages_with_path(path)
+        matching_packages = base.only_maven_packages_with_path(path, use_cte: group.present?)
         matching_packages = matching_packages.order_by_package_file if versionless_package?(matching_packages)
 
         matching_packages

app/models/packages/package.rb

@@ -141,9 +141,20 @@ class Packages::Package < ApplicationRecord
     where(project_id: projects)
   end
 
-  def self.only_maven_packages_with_path(path)
+  def self.only_maven_packages_with_path(path, use_cte: false)
+    if use_cte && Feature.enabled?(:maven_metadata_by_path_with_optimization_fence)
+      # This is an optimization fence which assumes that looking up the Metadatum record by path (globally)
+      # and then filter down the packages (by project or by group and subgroups) will be cheaper than
+      # looking up all packages within a project or group and filter them by path.
+
+      inner_query = Packages::Maven::Metadatum.where(path: path).select(:id, :package_id)
+      cte = Gitlab::SQL::CTE.new(:maven_metadata_by_path, inner_query)
+      with(cte.to_arel)
+        .joins('INNER JOIN maven_metadata_by_path ON maven_metadata_by_path.package_id=packages_packages.id')
+    else
       joins(:maven_metadatum).where(packages_maven_metadata: { path: path })
     end
+  end
 
   def self.by_name_and_file_name(name, file_name)
     with_name(name)

config/feature_flags/development/maven_metadata_by_path_with_optimization_fence.yml

+---
+name: maven_metadata_by_path_with_optimization_fence
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/57041
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/325460
+milestone: '13.11'
+type: development
+group: group::optimize
+default_enabled: false

spec/finders/packages/maven/package_finder_spec.rb

@@ -17,6 +17,7 @@ RSpec.describe ::Packages::Maven::PackageFinder do
     group.add_developer(user)
   end
 
+  shared_examples 'Packages::Maven::PackageFinder examples' do
     describe '#execute!' do
       subject { finder.execute! }
 
@@ -78,4 +79,27 @@ RSpec.describe ::Packages::Maven::PackageFinder do
         it { is_expected.to eq(package2) }
       end
     end
+  end
+
+  context 'when the maven_metadata_by_path_with_optimization_fence feature flag is off' do
+    before do
+      stub_feature_flags(maven_metadata_by_path_with_optimization_fence: false)
+    end
+
+    it_behaves_like 'Packages::Maven::PackageFinder examples'
+  end
+
+  context 'when the maven_metadata_by_path_with_optimization_fence feature flag is on' do
+    before do
+      stub_feature_flags(maven_metadata_by_path_with_optimization_fence: true)
+    end
+
+    it_behaves_like 'Packages::Maven::PackageFinder examples'
+
+    it 'uses CTE in the query' do
+      sql = described_class.new('some_path', user, group: group).send(:packages_with_path).to_sql
+
+      expect(sql).to include('WITH "maven_metadata_by_path" AS')
+    end
+  end
 end