Backport: Optimize slow pipelines.js response

Add changelog

app/models/ci/build.rb

@@ -48,12 +48,22 @@ module Ci
     delegate :trigger_short_token, to: :trigger_request, allow_nil: true
 
     ##
-    # The "environment" field for builds is a String, and is the unexpanded name!
+    # Since Gitlab 11.5, deployments records started being created right after
+    # `ci_builds` creation. We can look up a relevant `environment` through
+    # `deployment` relation today. This is much more efficient than expanding
+    # environment name with variables.
+    # (See more https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22380)
     #
+    # However, we have to still expand environment name if it's a stop action,
+    # because `deployment` persists information for start action only.
+    #
+    # We will follow up this by persisting expanded name in build metadata or
+    # persisting stop action in database.
     def persisted_environment
       return unless has_environment?
 
       strong_memoize(:persisted_environment) do
+        deployment&.environment ||
           Environment.find_by(name: expanded_environment_name, project: project)
       end
     end

changelogs/unreleased/use-deployment-relation-to-fetch-environment-ce.yml

+---
+title: Use deployment relation to get an environment name
+merge_request: 24890
+author:
+type: performance

spec/models/ci/build_spec.rb

@@ -1844,6 +1844,26 @@ describe Ci::Build do
     context 'when there is no environment' do
       it { is_expected.to be_nil }
     end
+
+    context 'when build has a start environment' do
+      let(:build) { create(:ci_build, :deploy_to_production, pipeline: pipeline) }
+
+      it 'does not expand environment name' do
+        expect(build).not_to receive(:expanded_environment_name)
+
+        subject
+      end
+    end
+
+    context 'when build has a stop environment' do
+      let(:build) { create(:ci_build, :stop_review_app, pipeline: pipeline) }
+
+      it 'expands environment name' do
+        expect(build).to receive(:expanded_environment_name)
+
+        subject
+      end
+    end
   end
 
   describe '#play' do