Remove ci_pipelines_for_merge_request_finder_new_cte feature flag

This commit removes the flag as it's deemed stable.

app/finders/ci/pipelines_for_merge_request_finder.rb

@@ -34,11 +34,7 @@ module Ci
 
         pipelines =
           if merge_request.persisted?
-            if Feature.enabled?(:ci_pipelines_for_merge_request_finder_new_cte, target_project)
             pipelines_using_cte
-            else
-              pipelines_using_legacy_cte
-            end
           else
             triggered_for_branch.for_sha(commit_shas)
           end
@@ -49,18 +45,6 @@ module Ci
 
     private
 
-    def pipelines_using_legacy_cte
-      cte = Gitlab::SQL::CTE.new(:shas, merge_request.all_commits.select(:sha))
-
-      source_sha_join = cte.table[:sha].eq(Ci::Pipeline.arel_table[:source_sha])
-      merged_result_pipelines = filter_by(triggered_by_merge_request, cte, source_sha_join)
-      detached_merge_request_pipelines = filter_by_sha(triggered_by_merge_request, cte)
-      pipelines_for_branch = filter_by_sha(triggered_for_branch, cte)
-
-      Ci::Pipeline.with(cte.to_arel) # rubocop: disable CodeReuse/ActiveRecord
-        .from_union([merged_result_pipelines, detached_merge_request_pipelines, pipelines_for_branch])
-    end
-
     def pipelines_using_cte
       cte = Gitlab::SQL::CTE.new(:shas, merge_request.all_commits.select(:sha))
 

changelogs/unreleased/remove-ci_pipelines_for_merge_request_finder_new_cte-feature-flag.yml

+---
+title: Improve the database query performance on the pipeline loading in merge requests
+merge_request: 50818
+author:
+type: performance

config/feature_flags/development/ci_pipelines_for_merge_request_finder_new_cte.yml

----
-name: ci_pipelines_for_merge_request_finder_new_cte
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/49083
-rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/291006
-milestone: '13.7'
-type: development
-group: group::continuous integration
-default_enabled: false

spec/finders/ci/pipelines_for_merge_request_finder_spec.rb

@@ -225,24 +225,6 @@ RSpec.describe Ci::PipelinesForMergeRequestFinder do
                     branch_pipeline_2,
                     branch_pipeline])
         end
-
-        context 'when ci_pipelines_for_merge_request_finder_new_cte feature flag is disabled' do
-          before do
-            stub_feature_flags(ci_pipelines_for_merge_request_finder_new_cte: false)
-          end
-
-          it 'returns only related merge request pipelines' do
-            expect(subject.all)
-              .to eq([detached_merge_request_pipeline,
-                      branch_pipeline_2,
-                      branch_pipeline])
-
-            expect(described_class.new(merge_request_2, nil).all)
-              .to eq([detached_merge_request_pipeline_2,
-                      branch_pipeline_2,
-                      branch_pipeline])
-          end
-        end
       end
 
       context 'when detached merge request pipeline is run on head ref of the merge request' do