Hide decomposed query behind a feature flag

https://gitlab.com/gitlab-org/gitlab/-/issues/341341

app/finders/ci/pipelines_for_merge_request_finder.rb

@@ -47,13 +47,49 @@ module Ci
 
     private
 
+    # rubocop: disable CodeReuse/ActiveRecord
+    def pipelines_using_cte
+      sha_relation = merge_request.all_commits.select(:sha).distinct
+
+      cte = Gitlab::SQL::CTE.new(:shas, sha_relation)
+
+      pipelines_for_merge_requests = triggered_by_merge_request
+      pipelines_for_branch = filter_by_sha(triggered_for_branch, cte)
+
+      Ci::Pipeline.with(cte.to_arel) # rubocop: disable CodeReuse/ActiveRecord
+        .from_union([pipelines_for_merge_requests, pipelines_for_branch])
+    end
+    # rubocop: enable CodeReuse/ActiveRecord
+
+    def filter_by_sha(pipelines, cte)
+      hex = Arel::Nodes::SqlLiteral.new("'hex'")
+      string_sha = Arel::Nodes::NamedFunction.new('encode', [cte.table[:sha], hex])
+      join_condition = string_sha.eq(Ci::Pipeline.arel_table[:sha])
+
+      filter_by(pipelines, cte, join_condition)
+    end
+
+    def filter_by(pipelines, cte, join_condition)
+      shas_table =
+        Ci::Pipeline.arel_table
+          .join(cte.table, Arel::Nodes::InnerJoin)
+          .on(join_condition)
+          .join_sources
+
+      pipelines.joins(shas_table) # rubocop: disable CodeReuse/ActiveRecord
+    end
+
     def all_pipelines_for_merge_request
+      if Feature.enabled?(:decomposed_ci_query_in_pipelines_for_merge_request_finder, source_project, default_enabled: :yaml)
+        pipelines_using_cte
+      else
         shas = merge_request.all_commit_shas
         pipelines_for_merge_request = triggered_by_merge_request
         pipelines_for_branch = triggered_for_branch.for_sha(shas)
 
         Ci::Pipeline.from_union([pipelines_for_merge_request, pipelines_for_branch])
       end
+    end
 
     # NOTE: this method returns only parent merge request pipelines.
     # Child merge request pipelines have a different source.

config/feature_flags/development/decomposed_ci_query_in_pipelines_for_merge_request_finder.yml

+---
+name: decomposed_ci_query_in_pipelines_for_merge_request_finder
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/68549
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/341341
+milestone: '14.4'
+type: development
+group: group::pipeline execution
+default_enabled: false

spec/finders/ci/pipelines_for_merge_request_finder_spec.rb

@@ -134,6 +134,7 @@ RSpec.describe Ci::PipelinesForMergeRequestFinder do
       end
     end
 
+    shared_examples 'returns all pipelines for merge request' do
       context 'when pipelines exist for the branch and merge request' do
         let(:source_ref) { 'feature' }
         let(:target_ref) { 'master' }
@@ -247,4 +248,15 @@ RSpec.describe Ci::PipelinesForMergeRequestFinder do
         end
       end
     end
+
+    it_behaves_like 'returns all pipelines for merge request'
+
+    context 'when `decomposed_ci_query_in_pipelines_for_merge_request_finder` feature flag disabled' do
+      before do
+        stub_feature_flags(decomposed_ci_query_in_pipelines_for_merge_request_finder: false)
+      end
+
+      it_behaves_like 'returns all pipelines for merge request'
+    end
+  end
 end